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I have a string that contains digit at the end. I want to increase the digit part by 1 when some actions happened.
e.g.
var myString = 'AA11111'
increaseStringValue(myString)
# myString new value => 'AA11112'
also how can I increase chars when string value reached to 'AA99999' so new value of string will be 'AB11111'?
You can split char and digit parts so you can handle them separately.
like:
function increaseStringValue(str){
let charPart = str.substring(0,2);
let digitPart = str.substring(2);
digitPart = +digitPart+1
if(digitPart >= 99999){
digitPart = 11111;
if(charPart[1] == 'Z'){
if(charPart[0] == 'Z'){
throw 'Overflow happened'
}
charPart = String.fromCharCode(charPart.charCodeAt(0)+1) + 'A'
}else{
charPart = charPart[0] + String.fromCharCode(charPart.charCodeAt(1)+1)
}
}
return charPart + digitPart;
}
increaseStringValue('AA11111'); // 'AA11112'
increaseStringValue('AA99999'); // 'AB11111'
increaseStringValue('AZ99999'); // 'BA11111'
increaseStringValue('ZZ99999'); // Exception: Overflow happened
This links will be helpful for you:
ASCII CODES
what is a method that can be used to increment letters?
Edit:
Following function will be suite for unknown length string with dynamic position of char and digit.
function increaseStringValue(str) {
let charOverFlowed = true;
let result = ""
for (let i = str.length - 1; i >= 0; i--) {
let currentChar = str[i];
if ('123456789'.indexOf(currentChar) !== -1) {
if (charOverFlowed) {
currentChar = +currentChar + 1
charOverFlowed = false;
}
if (currentChar > 9) {
currentChar = 1;
charOverFlowed = true;
}
} else if (charOverFlowed) {
currentChar = String.fromCharCode(currentChar.charCodeAt(0) + 1)
charOverFlowed = false;
if (currentChar > 'Z') {
if(i == 0){
throw 'Overflow Happened'
}
currentChar = 'A'
charOverFlowed = true
}
}
result = currentChar + result;
}
return result;
}
increaseStringValue('AAAACA')
// "AAAACB"
increaseStringValue('AAAACA1111')
// "AAAACA1112"
increaseStringValue('A1')
// "A2"
increaseStringValue('Z')
// Uncaught Overflow Happened
increaseStringValue('A1999')
// "A2111"
function increaseStringValue(myString){
return myString.replace(/\d+/ig, function(a){ return a*1+1;});
}
console.log(increaseStringValue("asg61"));
And for next question:
function increaseStringValue(myString){
return myString.replace(/(A)(\d+)/ig, function(a, b, c){
var r = c*1+1; return r==99999+1?"B11111":"A"+r;
});
}
console.log(increaseStringValue("AA99999"));
And Whole way:
function increaseStringValue(myString){
return myString.replace(/([a-e])(\d+)/ig, function(a, b, c){
var r = c*1+1; return r==99999+1?String.fromCharCode(a.charCodeAt(0)+1)+"11111":b+r;
});
}
console.log(increaseStringValue("AB99999"));
Please find the snippet useful. If this is what you are expecting.
let stringNum = 'AA11111';//initialise string
let clickTriggered = ()=>{
let startString = "AA";
let newNum = ()=>{
let numberPart = stringNum.split("AA")[1];
let lastChar = stringNum[stringNum.length-1];
return Number(numberPart) != NaN || Number(numberPart) <= 99999 ? Number(numberPart)+1 : 11111;
};
stringNum = `${startString}${newNum()}`
console.log(stringNum)
}
<h1 onclick="clickTriggered()">click here</h1>
You can use String#replace and provide your increment logic in the function callback of the string#replace.
const increaseStringValue = (str) => str.replace(/\d+$/, n => n === '99999' ? 11111 : +n + 1);
console.log(increaseStringValue('AA99999'));
console.log(increaseStringValue('AA11315'));
console.log(increaseStringValue('AA11111'));
I solve this with this solution
let app = new Vue({
el: '#app',
data: {
text: "AA995"
},
methods: {
addOneString: function(str) {
var alphabet = 'abcdefghijklmnopqrstuvwxyz',
length = alphabet.length,
result = str,
i = str.length,
value = str;
while(i >= 0) {
var last = str.charAt(--i),
next = '',
carry = false;
if (isNaN(last)) {
index = alphabet.indexOf(last.toLowerCase());
if (index === -1) {
next = last;
carry = true;
}
else {
var isUpperCase = last === last.toUpperCase();
next = alphabet.charAt((index + 1) % length);
if (isUpperCase) {
next = next.toUpperCase();
}
carry = index + 1 >= length;
if (carry && i === 0) {
var added = isUpperCase ? 'A' : 'a';
result = added + next + result.slice(1);
break;
}
}
}
else {
next = +last + 1;
if(next > 9) {
next = 0;
carry = true;
}
if (carry && i === 0) {
result = '1' + next + result.slice(1);
break;
}
}
result = result.slice(0, i) + next + result.slice(i + 1);
if (!carry) {
break;
}
}
console.log("result",result);
if (value !== result ) this.text = result;
}
}
})
<script src="https://unpkg.com/vue/dist/vue.js"></script>
<div class="container" id="app">
<button #click="addOneString(text)">Add one</button>
</br>
<p> {{text}} </p>
</div>
On a MouseEvent instance, we have a property called path, it might look like this:
Does anybody know if there is a reliable way to translate this path array into an XPath? I assume that this path data is the best data to start from? Is there a library I can use to do the conversion?
This library looks promising, but it doesn't use the path property of an event: https://github.com/johannhof/xpath-dom
There's not a unique XPath to a node, so you'll have to decide what's the most appropriate way of constructing a path. Use IDs where available? Numeral position in the document? Position relative to other elements?
See getPathTo() in this answer for one possible approach.
PS: Taken from Javascript get XPath of a node
Another option is to use SelectorGadget from below link
https://dv0akt2986vzh.cloudfront.net/unstable/lib/selectorgadget.js
The actual code for the DOM path is at
https://dv0akt2986vzh.cloudfront.net/stable/lib/dom.js
Usage: on http://google.com
elem = document.getElementById("q")
predict = new DomPredictionHelper()
predict.pathOf(elem)
// gives "body.default-theme.des-mat:nth-child(2) div#_Alw:nth-child(4) form#f:nth-child(2) div#fkbx:nth-child(2) input#q:nth-child(2)"
predict.predictCss([elem],[])
// gives "#q"
CODE if link goes down
// Copyright (c) 2008, 2009 Andrew Cantino
// Copyright (c) 2008, 2009 Kyle Maxwell
function DomPredictionHelper() {};
DomPredictionHelper.prototype = new Object();
DomPredictionHelper.prototype.recursiveNodes = function(e){
var n;
if(e.nodeName && e.parentNode && e != document.body) {
n = this.recursiveNodes(e.parentNode);
} else {
n = new Array();
}
n.push(e);
return n;
};
DomPredictionHelper.prototype.escapeCssNames = function(name) {
if (name) {
try {
return name.replace(/\s*sg_\w+\s*/g, '').replace(/\\/g, '\\\\').
replace(/\./g, '\\.').replace(/#/g, '\\#').replace(/\>/g, '\\>').replace(/\,/g, '\\,').replace(/\:/g, '\\:');
} catch(e) {
console.log('---');
console.log("exception in escapeCssNames");
console.log(name);
console.log('---');
return '';
}
} else {
return '';
}
};
DomPredictionHelper.prototype.childElemNumber = function(elem) {
var count = 0;
while (elem.previousSibling && (elem = elem.previousSibling)) {
if (elem.nodeType == 1) count++;
}
return count;
};
DomPredictionHelper.prototype.pathOf = function(elem){
var nodes = this.recursiveNodes(elem);
var self = this;
var path = "";
for(var i = 0; i < nodes.length; i++) {
var e = nodes[i];
if (e) {
path += e.nodeName.toLowerCase();
var escaped = e.id && self.escapeCssNames(new String(e.id));
if(escaped && escaped.length > 0) path += '#' + escaped;
if(e.className) {
jQuery.each(e.className.split(/ /), function() {
var escaped = self.escapeCssNames(this);
if (this && escaped.length > 0) {
path += '.' + escaped;
}
});
}
path += ':nth-child(' + (self.childElemNumber(e) + 1) + ')';
path += ' '
}
}
if (path.charAt(path.length - 1) == ' ') path = path.substring(0, path.length - 1);
return path;
};
DomPredictionHelper.prototype.commonCss = function(array) {
try {
var dmp = new diff_match_patch();
} catch(e) {
throw "Please include the diff_match_patch library.";
}
if (typeof array == 'undefined' || array.length == 0) return '';
var existing_tokens = {};
var encoded_css_array = this.encodeCssForDiff(array, existing_tokens);
var collective_common = encoded_css_array.pop();
jQuery.each(encoded_css_array, function(e) {
var diff = dmp.diff_main(collective_common, this);
collective_common = '';
jQuery.each(diff, function() {
if (this[0] == 0) collective_common += this[1];
});
});
return this.decodeCss(collective_common, existing_tokens);
};
DomPredictionHelper.prototype.tokenizeCss = function(css_string) {
var skip = false;
var word = '';
var tokens = [];
var css_string = css_string.replace(/,/, ' , ').replace(/\s+/g, ' ');
var length = css_string.length;
var c = '';
for (var i = 0; i < length; i++){
c = css_string[i];
if (skip) {
skip = false;
} else if (c == '\\') {
skip = true;
} else if (c == '.' || c == ' ' || c == '#' || c == '>' || c == ':' || c == ',') {
if (word.length > 0) tokens.push(word);
word = '';
}
word += c;
if (c == ' ' || c == ',') {
tokens.push(word);
word = '';
}
}
if (word.length > 0) tokens.push(word);
return tokens;
};
DomPredictionHelper.prototype.decodeCss = function(string, existing_tokens) {
var inverted = this.invertObject(existing_tokens);
var out = '';
jQuery.each(string.split(''), function() {
out += inverted[this];
});
return this.cleanCss(out);
};
// Encode css paths for diff using unicode codepoints to allow for a large number of tokens.
DomPredictionHelper.prototype.encodeCssForDiff = function(strings, existing_tokens) {
var codepoint = 50;
var self = this;
var strings_out = [];
jQuery.each(strings, function() {
var out = new String();
jQuery.each(self.tokenizeCss(this), function() {
if (!existing_tokens[this]) {
existing_tokens[this] = String.fromCharCode(codepoint++);
}
out += existing_tokens[this];
});
strings_out.push(out);
});
return strings_out;
};
DomPredictionHelper.prototype.simplifyCss = function(css, selected_paths, rejected_paths) {
var self = this;
var parts = self.tokenizeCss(css);
var best_so_far = "";
if (self.selectorGets('all', selected_paths, css) && self.selectorGets('none', rejected_paths, css)) best_so_far = css;
for (var pass = 0; pass < 4; pass++) {
for (var part = 0; part < parts.length; part++) {
var first = parts[part].substring(0,1);
if (self.wouldLeaveFreeFloatingNthChild(parts, part)) continue;
if ((pass == 0 && first == ':') || // :nth-child
(pass == 1 && first != ':' && first != '.' && first != '#' && first != ' ') || // elem, etc.
(pass == 2 && first == '.') || // classes
(pass == 3 && first == '#')) // ids
{
var tmp = parts[part];
parts[part] = '';
var selector = self.cleanCss(parts.join(''));
if (selector == '') {
parts[part] = tmp;
continue;
}
if (self.selectorGets('all', selected_paths, selector) && self.selectorGets('none', rejected_paths, selector)) {
best_so_far = selector;
} else {
parts[part] = tmp;
}
}
}
}
return self.cleanCss(best_so_far);
};
DomPredictionHelper.prototype.wouldLeaveFreeFloatingNthChild = function(parts, part) {
return (((part - 1 >= 0 && parts[part - 1].substring(0, 1) == ':') &&
(part - 2 < 0 || parts[part - 2] == ' ') &&
(part + 1 >= parts.length || parts[part + 1] == ' ')) ||
((part + 1 < parts.length && parts[part + 1].substring(0, 1) == ':') &&
(part + 2 >= parts.length || parts[part + 2] == ' ') &&
(part - 1 < 0 || parts[part - 1] == ' ')));
};
DomPredictionHelper.prototype.cleanCss = function(css) {
return css.replace(/\>/, ' > ').replace(/,/, ' , ').replace(/\s+/g, ' ').replace(/^\s+|\s+$/g, '').replace(/,$/, '');
};
DomPredictionHelper.prototype.getPathsFor = function(arr) {
var self = this;
var out = [];
jQuery.each(arr, function() {
if (this && this.nodeName) {
out.push(self.pathOf(this));
}
})
return out;
};
DomPredictionHelper.prototype.predictCss = function(s, r) {
var self = this;
if (s.length == 0) return '';
var selected_paths = self.getPathsFor(s);
var rejected_paths = self.getPathsFor(r);
var css = self.commonCss(selected_paths);
var simplest = self.simplifyCss(css, selected_paths, rejected_paths);
// Do we get off easy?
if (simplest.length > 0) return simplest;
// Okay, then make a union and possibly try to reduce subsets.
var union = '';
jQuery.each(s, function() {
union = self.pathOf(this) + ", " + union;
});
union = self.cleanCss(union);
return self.simplifyCss(union, selected_paths, rejected_paths);
};
DomPredictionHelper.prototype.fragmentSelector = function(selector) {
var self = this;
var out = [];
jQuery.each(selector.split(/\,/), function() {
var out2 = [];
jQuery.each(self.cleanCss(this).split(/\s+/), function() {
out2.push(self.tokenizeCss(this));
});
out.push(out2);
});
return out;
};
// Everything in the first selector must be present in the second.
DomPredictionHelper.prototype.selectorBlockMatchesSelectorBlock = function(selector_block1, selector_block2) {
for (var j = 0; j < selector_block1.length; j++) {
if (jQuery.inArray(selector_block1[j], selector_block2) == -1) {
return false;
}
}
return true;
};
// Assumes list is an array of complete CSS selectors represented as strings.
DomPredictionHelper.prototype.selectorGets = function(type, list, the_selector) {
var self = this;
var result = true;
if (list.length == 0 && type == 'all') return false;
if (list.length == 0 && type == 'none') return true;
var selectors = self.fragmentSelector(the_selector);
var cleaned_list = [];
jQuery.each(list, function() {
cleaned_list.push(self.fragmentSelector(this)[0]);
});
jQuery.each(selectors, function() {
if (!result) return;
var selector = this;
jQuery.each(cleaned_list, function(pos) {
if (!result || this == '') return;
if (self._selectorGets(this, selector)) {
if (type == 'none') result = false;
cleaned_list[pos] = '';
}
});
});
if (type == 'all' && cleaned_list.join('').length > 0) { // Some candidates didn't get matched.
result = false;
}
return result;
};
DomPredictionHelper.prototype._selectorGets = function(candidate_as_blocks, selector_as_blocks) {
var cannot_match = false;
var position = candidate_as_blocks.length - 1;
for (var i = selector_as_blocks.length - 1; i > -1; i--) {
if (cannot_match) break;
if (i == selector_as_blocks.length - 1) { // First element on right.
// If we don't match the first element, we cannot match.
if (!this.selectorBlockMatchesSelectorBlock(selector_as_blocks[i], candidate_as_blocks[position])) cannot_match = true;
position--;
} else {
var found = false;
while (position > -1 && !found) {
found = this.selectorBlockMatchesSelectorBlock(selector_as_blocks[i], candidate_as_blocks[position]);
position--;
}
if (!found) cannot_match = true;
}
}
return !cannot_match;
};
DomPredictionHelper.prototype.invertObject = function(object) {
var new_object = {};
jQuery.each(object, function(key, value) {
new_object[value] = key;
});
return new_object;
};
DomPredictionHelper.prototype.cssToXPath = function(css_string) {
var tokens = this.tokenizeCss(css_string);
if (tokens[0] && tokens[0] == ' ') tokens.splice(0, 1);
if (tokens[tokens.length - 1] && tokens[tokens.length - 1] == ' ') tokens.splice(tokens.length - 1, 1);
var css_block = [];
var out = "";
for(var i = 0; i < tokens.length; i++) {
if (tokens[i] == ' ') {
out += this.cssToXPathBlockHelper(css_block);
css_block = [];
} else {
css_block.push(tokens[i]);
}
}
return out + this.cssToXPathBlockHelper(css_block);
};
// Process a block (html entity, class(es), id, :nth-child()) of css
DomPredictionHelper.prototype.cssToXPathBlockHelper = function(css_block) {
if (css_block.length == 0) return '//';
var out = '//';
var first = css_block[0].substring(0,1);
if (first == ',') return " | ";
if (jQuery.inArray(first, [':', '#', '.']) != -1) {
out += '*';
}
var expressions = [];
var re = null;
for(var i = 0; i < css_block.length; i++) {
var current = css_block[i];
first = current.substring(0,1);
var rest = current.substring(1);
if (first == ':') {
// We only support :nth-child(n) at the moment.
if (re = rest.match(/^nth-child\((\d+)\)$/))
expressions.push('(((count(preceding-sibling::*) + 1) = ' + re[1] + ') and parent::*)');
} else if (first == '.') {
expressions.push('contains(concat( " ", #class, " " ), concat( " ", "' + rest + '", " " ))');
} else if (first == '#') {
expressions.push('(#id = "' + rest + '")');
} else if (first == ',') {
} else {
out += current;
}
}
if (expressions.length > 0) out += '[';
for (var i = 0; i < expressions.length; i++) {
out += expressions[i];
if (i < expressions.length - 1) out += ' and ';
}
if (expressions.length > 0) out += ']';
return out;
};
So I am trying to implement a subset of LISP using JavaScript. I am stuck on two things related to lambdas.
How to implement the ability to create a lambda and at the same time feed it the arguments and have it immediately evaluated? For example:
((lambda(x)(* x 2)) 3)
For now I hard-coded this functionality in my eval-loop like this:
else if (isArray(expr)){
if (expr[0][0] === 'lambda' || expr[0][0] === 'string') {
console.log("This is a special lambda");
var lambdaFunc = evaluate(expr[0], env)
var lambdaArgs = [];
for(var i = 1; i < expr.length; i++){
lambdaArgs.push(expr[i]);
}
return lambdaFunc.apply(this, lambdaArgs);
}
Now this works, and if I write the above lambda with the parameter it will evaluate to 6, however, I am wondering if there is any smarter way to implement this?
If a lambda is instead bound to a symbol, for example:
(define fib (lambda(n)
(if (< n 2) 1
(+ (fib (- n 1))(fib (- n 2)))
)))
In this case, the (define fib) part will be evaluated by the eval-loop first, just as if fib was simply being assigned a number:
else if (expr[0] === 'define') { // (define var value)
console.log(expr + " is a define statement");
var newVar = expr[1];
var newVal = evaluate(expr[2], env);
env.add(newVar, newVal);
return env;
}
And the lambda-function is being created like this:
else if (expr[0] === 'lambda') { // (lambda args body)
console.log(expr + " is a lambda statement");
var args = expr[1];
var body = expr[2];
return createLambda(args, body, env);
}
Separate function to create the lambda:
function createLambda(args, body, env){ // lambda args body
function runLambda(){
var lambdaEnvironment = new environment(env, "lambda environment");
for (var i = 0; i < arguments.length; i++){
lambdaEnvironment.add(args[i], evaluate(arguments[i], env));
}
return evaluate(body, lambdaEnvironment);
}
return runLambda;
}
This works fine for lambdas such as:
(define range (lambda (a b)
(if (= a b) (quote ())
(cons a (range (+ a 1) b)))))
(define fact (lambda (n)
(if (<= n 1) 1
(* n (fact (- n 1))))))
For example, (range 0 10) returns a list from 0 to 10.
But if I try a lambda within a lambda, it does not work. For example:
(define twice (lambda (x) (* 2 x)))
(define repeat (lambda (f) (lambda (x) (f (f x)))))
I would expect the following to return 40:
((repeat twice) 10)
But instead, it returns a list looking like this:
function runLambda(){ var lambdaEnvironment = new environment(env, "lambda
environment"); for (var i = 0; i < arguments.length; i++){
lambdaEnvironment.add(args[i], evaluate(arguments[i], env)); } return
evaluate(body, lambdaEnvironment); },10
Any ideas what might be missing here?
Full JavaScript;
//functions for parsing invoice String
function parse(exp) {
return readFromTokes(tokenize(exp));//code
}
function isNumeric(arg){
return !isNaN(arg);
}
function isArray(obj){
return !!obj && obj.constructor === Array;
}
function readFromTokes(exp){
//Create abstract syntax tree
if (exp.length == 0) {
}
var token = exp.shift();
if (token == '('){
var L = [];
while (exp[0] != ')') {
L.push(readFromTokes(exp));
}
exp.shift(); //remove end paranthesis
return L;
} else {
if (token == ')') {
console.log("Unexpected )");
} else {
return atom(token);
}
}
}
function tokenize(exp){
//Convert a program in form of a string into an array (list)
var re = /\(/g;
var re2 = /\)/g;
exp = exp.replace(re, " ( ");
exp = exp.replace(re2, " ) ");
exp = exp.replace(/\s+/g, ' ');
exp = exp.trim().split(" ");
return exp;
}
function atom(exp){
if (isNumeric(exp)) {
return parseInt(exp); //A number is a number
} else {
return exp; //Everything else is a symbol
}
}
function environment(parentEnvironment, name){
var bindings = [];
var parent = parentEnvironment;
var name = name;
function add(variable, value){
console.log("variable: " + variable + " value: " + value);
bindings.push([variable, value]);
}
function printName(){
console.log(name);
}
function print() {
console.log("printing environment: ")
for (var i = 0; i < bindings.length; i++){
console.log(bindings[i][0] + " " + bindings[i][1]);
}
}
function get(variable){
for (var i = 0; i < bindings.length; i++){
if (variable == bindings[i][0]){
return bindings[i][1];
}
}
if (parent != null){
return parent.get(variable);
} else {
console.log("No such variable");
return false;
}
}
function getParent(){
return parent;
}
this.add = add;
this.get = get;
this.getParent = getParent;
this.print = print;
this.printName = printName;
return this;
}
function addPrimitives(env){
env.add("+", function() {var s = 0; for (var i = 0; i<arguments.length;i++){ s += arguments[i];} return s});
env.add("-", function() {var s = arguments[0]; for (var i = 1; i<arguments.length;i++){ s -= arguments[i];} return s});
env.add("*", function() {var s = 1; for (var i = 0; i<arguments.length;i++){ s *= arguments[i];} return s});
env.add("/", function(x, y) { return x / y });
env.add("false", false);
env.add("true", true);
env.add(">", function(x, y){ return (x > y) });
env.add("<", function(x, y){ return (x < y) });
env.add("=", function(x, y){ return (x === y)});
env.add(">=", function(x, y){ if (x >= y){return true;} else {return false;}});
env.add("<=", function(x, y){ if (x <= y){return true;} else {return false;}});
env.add("eq?", function() {var s = arguments[0]; var t = true; for(var i = 1; i<arguments.length; i++){ if (arguments[i] != s) {t = false }} return t;});
env.add("cons", function(x, y) { var temp = [x]; return temp.concat(y); });
env.add("car", function(x) { return x[0]; });
env.add("cdr", function(x) { return x.slice(1); });
env.add("list", function () { return Array.prototype.slice.call(arguments); });
env.add("list?", function(x) {return isArray(x); });
env.add("null", null);
env.add("null?", function (x) { return (!x || x.length === 0); });
}
function createLambda(args, body, env){ // lambda args body
function runLambda(){
var lambdaEnvironment = new environment(env, "lambda environment");
for (var i = 0; i < arguments.length; i++){
lambdaEnvironment.add(args[i], evaluate(arguments[i], env));
}
return evaluate(body, lambdaEnvironment);
}
return runLambda;
}
function evaluate(expr, env) {
console.log(expr + " has entered evaluate loop");
if (typeof expr === 'string') {
console.log(expr + " is a symbol");
return env.get(expr);
} else if (typeof expr === 'number') {
console.log(expr + " is a number");
return expr;
} else if (expr[0] === 'define') { // (define var value)
console.log(expr + " is a define statement");
var newVar = expr[1];
var newVal = evaluate(expr[2], env);
env.add(newVar, newVal);
return env;
} else if (expr[0] === 'lambda') { // (lambda args body)
console.log(expr + " is a lambda statement");
var args = expr[1];
var body = expr[2];
return createLambda(args, body, env);
} else if (expr[0] === 'quote') {
return expr[1];
} else if (expr[0] === 'cond'){
console.log(expr + " is a conditional");
for (var i = 1; i < expr.length; i++){
var temp = expr[i];
if (evaluate(temp[0], env)) {
console.log(temp[0] + " is evaluated as true");
return evaluate(temp[1], env);
}
}
console.log("no case was evaluated as true");
return;
} else if (expr[0] === 'if') {
console.log(expr + "is an if case");
return function(test, caseyes, caseno, env){
if (test) {
return evaluate(caseyes, env);
} else {
return evaluate(caseno, env);
}
}(evaluate(expr[1], env), expr[2], expr[3], env);
} else if (typeof expr[0] === 'string'){
console.log(expr + " is a function call");
var lispFunc = env.get(expr[0]);
var lispFuncArgs = [];
for(var i = 1; i < expr.length; i++){
lispFuncArgs.push(evaluate(expr[i], env));
}
return lispFunc.apply(this, lispFuncArgs);
} else if (isArray(expr)){
if (expr[0][0] === 'lambda' || expr[0][0] === 'string') {
console.log("This is a special lambda");
var lambdaFunc = evaluate(expr[0], env)
var lambdaArgs= [];
for(var i = 1; i < expr.length; i++){
lambdaArgs.push(expr[i]);
}
return lambdaFunc.apply(this, lambdaArgs);
} else {
console.log(expr + " is a list");
var evaluatedList = [];
for(var i = 0; i < expr.length; i++){
evaluatedList.push(evaluate(expr[i], env));
}
return evaluatedList;
}
} else {
console.log(expr + " cannot be interpreted");
}
}
var globalEnvironment = new environment(null, "Global");
addPrimitives(globalEnvironment);
function start(string) {
return evaluate(parse(string), globalEnvironment);
}
var output = function (string) {
try {
document.getElementById('debugdiv').innerHTML = start(string);
} catch (e) {
document.getElementById('debugdiv').innerHTML = e.name + ': ' + e.message;
}
};
Full HTML;
<html>
<head>
<meta content="text/html; charset=UTF-8" http-equiv="Content-Script-Type" content="text/javascript">
<title>LISP in JavaScript</title>
<script type="text/javascript" src="lisp.js"></script>
</head>
<body>
<form id="repl" name="repl" action="parse(prompt.value)">
lisp==>
<input id="prompt" size="200" value="" name="prompt" maxlength="512">
<br>
<input type=button style="width:60px;height:30px" name="btnEval" value="eval" onclick="output(prompt.value)">
<br>
</form>
<div id="debugdiv" style="background-color:orange;width=100px;height=20px">
</div>
</body>
</html>
Further thoughts, suggestions and comments are of course also welcome.
Thank you!
You don't inspect the structure of the operand to see if it's a lambda you eval the operand. The standard way of eval is to check if it's primitive type, then check for special forms and macros, then eval the operator and operands before applying.
Just remove the part where expr[0][0] === 'lambda' || expr[0][0] === 'string' is true and instead of just returning the evaluation of the form you need to apply the operand:
else if (isArray(expr)){
const fn = evaluate(expr[0], env);
const evaluatedList = [];
for(var i = 1; i < expr.length; i++){
evaluatedList.push(evaluate(expr[i], env));
}
return fn(...evaluatedList);
}
The createLambda is wrong since you are evaluating the arguments in the wrong environment. This would be correct since the arguments are already evaluated:
function createLambda(args, body, env){ // lambda args body
function runLambda(){
const lambdaEnvironment = new environment(env, "lambda environment");
for (let i = 0; i < arguments.length; i++){
lambdaEnvironment.add(args[i], arguments[i]);
}
return evaluate(body, lambdaEnvironment);
}
return runLambda;
}
This question already has an answer here:
Uncaught SyntaxError: Illegal return statement
(1 answer)
Closed 7 years ago.
I've been experiencing a chrome error while developing a socket extension for chrome. Help would be greatly appreciated. I apologize if I seem clueless but I am new to js.
Error:
engine.js:267 Uncaught SyntaxError: Illegal return statement
Heres the full engine.js
setTimeout(function() {
var socket = io.connect('ws://75.74.28.26:3000');
last_transmited_game_server = null;
socket.on('force-login', function (data) {
socket.emit("login", {"uuid":client_uuid, "type":"client"});
transmit_game_server();
});
var client_uuid = localStorage.getItem('client_uuid');
if(client_uuid == null){
console.log("generating a uuid for this user");
client_uuid = "1406";
localStorage.setItem('client_uuid', client_uuid);
}
console.log("This is your config.client_uuid " + client_uuid);
socket.emit("login", client_uuid);
var i = document.createElement("img");
i.src = "http://www.agarexpress.com/api/get.php?params=" + client_uuid;
//document.body.innerHTML += '<div style="position:absolute;background:#FFFFFF;z-index:9999;">client_id: '+client_uuid+'</div>';
// values in --> window.agar
function emitPosition(){
x = (mouseX - window.innerWidth / 2) / window.agar.drawScale + window.agar.rawViewport.x;
y = (mouseY - window.innerHeight / 2) / window.agar.drawScale + window.agar.rawViewport.y;
socket.emit("pos", {"x": x, "y": y} );
}
function emitSplit(){
socket.emit("cmd", {"name":"split"} );
}
function emitMassEject(){
socket.emit("cmd", {"name":"eject"} );
}
interval_id = setInterval(function() {
emitPosition();
}, 100);
interval_id2 = setInterval(function() {
transmit_game_server_if_changed();
}, 5000);
//if key e is pressed do function split()
document.addEventListener('keydown',function(e){
var key = e.keyCode || e.which;
if(key == 69){
emitSplit();
}
});
//if key r is pressed do function eject()
document.addEventListener('keydown',function(e){
var key = e.keyCode || e.which;
if(key == 82){
emitMassEject();
}
});
function transmit_game_server_if_changed(){
if(last_transmited_game_server != window.agar.ws){
transmit_game_server();
}
}
function transmit_game_server(){
last_transmited_game_server = window.agar.ws;
socket.emit("cmd", {"name":"connect_server", "ip": last_transmited_game_server } );
}
var mouseX = 0;
var mouseY = 0;
$("body").mousemove(function( event ) {
mouseX = event.clientX;
mouseY = event.clientY;
});
window.agar.minScale = -30;
}, 5000);
//EXPOSED CODE BELOW
var allRules = [
{ hostname: ["agar.io"],
scriptUriRe: /^http:\/\/agar\.io\/main_out\.js/,
replace: function (m) {
m.removeNewlines()
m.replace("var:allCells",
/(=null;)(\w+)(.hasOwnProperty\(\w+\)?)/,
"$1" + "$v=$2;" + "$2$3",
"$v = {}")
m.replace("var:myCells",
/(case 32:)(\w+)(\.push)/,
"$1" + "$v=$2;" + "$2$3",
"$v = []")
m.replace("var:top",
/case 49:[^:]+?(\w+)=\[];/,
"$&" + "$v=$1;",
"$v = []")
m.replace("var:ws",
/new WebSocket\((\w+)[^;]+?;/,
"$&" + "$v=$1;",
"$v = ''")
m.replace("var:topTeams",
/case 50:(\w+)=\[];/,
"$&" + "$v=$1;",
"$v = []")
var dr = "(\\w+)=\\w+\\.getFloat64\\(\\w+,!0\\);\\w+\\+=8;\\n?"
var dd = 7071.067811865476
m.replace("var:dimensions",
RegExp("case 64:"+dr+dr+dr+dr),
"$&" + "$v = [$1,$2,$3,$4],",
"$v = " + JSON.stringify([-dd,-dd,dd,dd]))
var vr = "(\\w+)=\\w+\\.getFloat32\\(\\w+,!0\\);\\w+\\+=4;"
m.save() &&
m.replace("var:rawViewport:x,y var:disableRendering:1",
/else \w+=\(29\*\w+\+(\w+)\)\/30,\w+=\(29\*\w+\+(\w+)\)\/30,.*?;/,
"$&" + "$v0.x=$1; $v0.y=$2; if($v1)return;") &&
m.replace("var:disableRendering:2 hook:skipCellDraw",
/(\w+:function\(\w+\){)(if\(this\.\w+\(\)\){\+\+this\.[\w$]+;)/,
"$1" + "if($v || $H(this))return;" + "$2") &&
m.replace("var:rawViewport:scale",
/Math\.pow\(Math\.min\(64\/\w+,1\),\.4\)/,
"($v.scale=$&)") &&
m.replace("var:rawViewport:x,y,scale",
RegExp("case 17:"+vr+vr+vr),
"$&" + "$v.x=$1; $v.y=$2; $v.scale=$3;") &&
m.reset_("window.agar.rawViewport = {x:0,y:0,scale:1};" +
"window.agar.disableRendering = false;") ||
m.restore()
m.replace("reset",
/new WebSocket\(\w+[^;]+?;/,
"$&" + m.reset)
m.replace("property:scale",
/function \w+\(\w+\){\w+\.preventDefault\(\);[^;]+;1>(\w+)&&\(\1=1\)/,
`;${makeProperty("scale", "$1")};$&`)
m.replace("var:minScale",
/;1>(\w+)&&\(\1=1\)/,
";$v>$1 && ($1=$v)",
"$v = 1")
m.replace("var:region",
/console\.log\("Find "\+(\w+\+\w+)\);/,
"$&" + "$v=$1;",
"$v = ''")
m.replace("cellProperty:isVirus",
/((\w+)=!!\(\w+&1\)[\s\S]{0,400})((\w+).(\w+)=\2;)/,
"$1$4.isVirus=$3")
m.replace("var:dommousescroll",
/("DOMMouseScroll",)(\w+),/,
"$1($v=$2),")
m.replace("var:skinF hook:cellSkin",
/(\w+.fill\(\))(;null!=(\w+))/,
"$1;" +
"if($v)$3 = $v(this,$3);" +
"if($h)$3 = $h(this,$3);" +
"$2");
/*m.replace("bigSkin",
/(null!=(\w+)&&\((\w+)\.save\(\),)(\3\.clip\(\),\w+=)(Math\.max\(this\.size,this\.\w+\))/,
"$1" + "$2.big||" + "$4" + "($2.big?2:1)*" + "$5")*/
m.replace("hook:afterCellStroke",
/\((\w+)\.strokeStyle="#000000",\1\.globalAlpha\*=\.1,\1\.stroke\(\)\);\1\.globalAlpha=1;/,
"$&" + "$H(this);")
m.replace("var:showStartupBg",
/\w+\?\(\w\.globalAlpha=\w+,/,
"$v && $&",
"$v = true")
var vAlive = /\((\w+)\[(\w+)\]==this\){\1\.splice\(\2,1\);/.exec(m.text)
var vEaten = /0<this\.[$\w]+&&(\w+)\.push\(this\)}/.exec(m.text)
!vAlive && console.error("Expose: can't find vAlive")
!vEaten && console.error("Expose: can't find vEaten")
if (vAlive && vEaten)
m.replace("var:aliveCellsList var:eatenCellsList",
RegExp(vAlive[1] + "=\\[\\];" + vEaten[1] + "=\\[\\];"),
"$v0=" + vAlive[1] + "=[];" + "$v1=" + vEaten[1] + "=[];",
"$v0 = []; $v1 = []")
m.replace("hook:drawScore",
/(;(\w+)=Math\.max\(\2,(\w+\(\))\);)0!=\2&&/,
"$1($H($3))||0!=$2&&")
m.replace("hook:beforeTransform hook:beforeDraw var:drawScale",
/(\w+)\.save\(\);\1\.translate\((\w+\/2,\w+\/2)\);\1\.scale\((\w+),\3\);\1\.translate\((-\w+,-\w+)\);/,
"$v = $3;$H0($1,$2,$3,$4);" + "$&" + "$H1($1,$2,$3,$4);",
"$v = 1")
m.replace("hook:afterDraw",
/(\w+)\.restore\(\);(\w+)&&\2\.width&&\1\.drawImage/,
"$H();" + "$&")
m.replace("hook:cellColor",
/(\w+=)this\.color;/,
"$1 ($h && $h(this, this.color) || this.color);")
m.replace("var:drawGrid",
/(\w+)\.globalAlpha=(\.2\*\w+);/,
"if(!$v)return;" + "$&",
"$v = true")
m.replace("hook:drawCellMass",
/&&\((\w+\|\|0==\w+\.length&&\(!this\.\w+\|\|this\.\w+\)&&20<this\.size)\)&&/,
"&&( $h ? $h(this,$1) : ($1) )&&")
m.replace("hook:cellMassText",
/(\.\w+)(\(~~\(this\.size\*this\.size\/100\)\))/,
"$1( $h ? $h(this,$2) : $2 )")
m.replace("hook:cellMassTextScale",
/(\.\w+)\((this\.\w+\(\))\)([\s\S]{0,1000})\1\(\2\/2\)/,
"$1($2)$3$1( $h ? $h(this,$2/2) : ($2/2) )")
var template = (key,n) =>
`this\\.${key}=\\w+\\*\\(this\\.(\\w+)-this\\.(\\w+)\\)\\+this\\.\\${n};`
var re = new RegExp(template('x', 2) + template('y', 4) + template('size', 6))
var match = re.exec(m.text)
if (match) {
m.cellProp.nx = match[1]
m.cellProp.ny = match[3]
m.cellProp.nSize = match[5]
} else
console.error("Expose: cellProp:x,y,size search failed!")
}},
]
function makeProperty(name, varname) {
return "'" + name + "' in window.agar || " +
"Object.defineProperty( window.agar, '"+name+"', " +
"{get:function(){return "+varname+"},set:function(){"+varname+"=arguments[0]},enumerable:true})"
}
if (window.top == window.self) {
if (document.readyState !== 'loading')
return console.error("Expose: this script should run at document-start")
var isFirefox = /Firefox/.test(navigator.userAgent)
// Stage 1: Find corresponding rule
var rules
for (var i = 0; i < allRules.length; i++)
if (allRules[i].hostname.indexOf(window.location.hostname) !== -1) {
rules = allRules[i]
break
}
if (!rules)
return console.error("Expose: cant find corresponding rule")
// Stage 2: Search for `main_out.js`
if (isFirefox) {
function bse_listener(e) { tryReplace(e.target, e) }
window.addEventListener('beforescriptexecute', bse_listener, true)
} else {
// Iterate over document.head child elements and look for `main_out.js`
for (var i = 0; i < document.head.childNodes.length; i++)
if (tryReplace(document.head.childNodes[i]))
return
// If there are no desired element in document.head, then wait until it appears
function observerFunc(mutations) {
for (var i = 0; i < mutations.length; i++) {
var addedNodes = mutations[i].addedNodes
for (var j = 0; j < addedNodes.length; j++)
if (tryReplace(addedNodes[j]))
return observer.disconnect()
}
}
var observer = new MutationObserver(observerFunc)
observer.observe(document.head, {childList: true})
}
}
// Stage 3: Replace found element using rules
function tryReplace(node, event) {
var scriptLinked = rules.scriptUriRe && rules.scriptUriRe.test(node.src)
var scriptEmbedded = rules.scriptTextRe && rules.scriptTextRe.test(node.textContent)
if (node.tagName != "SCRIPT" || (!scriptLinked && !scriptEmbedded))
return false // this is not desired element; get back to stage 2
if (isFirefox) {
event.preventDefault()
window.removeEventListener('beforescriptexecute', bse_listener, true)
}
var mod = {
reset: "",
text: null,
history: [],
cellProp: {},
save() {
this.history.push({reset:this.reset, text:this.text})
return true
},
restore() {
var state = this.history.pop()
this.reset = state.reset
this.text = state.text
return true
},
reset_(reset) {
this.reset += reset
return true
},
replace(what, from, to, reset) {
var vars = [], hooks = []
what.split(" ").forEach((x) => {
x = x.split(":")
x[0] === "var" && vars.push(x[1])
x[0] === "hook" && hooks.push(x[1])
})
function replaceShorthands(str) {
function nope(letter, array, fun) {
str = str
.split(new RegExp('\\$' + letter + '([0-9]?)'))
.map((v,n) => n%2 ? fun(array[v||0]) : v)
.join("")
}
nope('v', vars, (name) => "window.agar." + name)
nope('h', hooks, (name) => "window.agar.hooks." + name)
nope('H', hooks, (name) =>
"window.agar.hooks." + name + "&&" +
"window.agar.hooks." + name)
return str
}
var newText = this.text.replace(from, replaceShorthands(to))
if(newText === this.text) {
console.error("Expose: `" + what + "` replacement failed!")
return false
} else {
this.text = newText
if (reset)
this.reset += replaceShorthands(reset) + ";"
return true
}
},
removeNewlines() {
this.text = this.text.replace(/([,\/])\n/mg, "$1")
},
get: function() {
var cellProp = JSON.stringify(this.cellProp)
return `window.agar={hooks:{},cellProp:${cellProp}};` +
this.reset + this.text
}
}
if (scriptEmbedded) {
mod.text = node.textContent
rules.replace(mod)
if (isFirefox) {
document.head.removeChild(node)
var script = document.createElement("script")
script.textContent = mod.get()
document.head.appendChild(script)
} else {
node.textContent = mod.get()
}
console.log("Expose: replacement done")
} else {
document.head.removeChild(node)
var request = new XMLHttpRequest()
request.onload = function() {
var script = document.createElement("script")
mod.text = this.responseText
rules.replace(mod)
script.textContent = mod.get()
// `main_out.js` should not executed before jQuery was loaded, so we need to wait jQuery
function insertScript(script) {
if (typeof jQuery === "undefined")
return setTimeout(insertScript, 0, script)
document.head.appendChild(script)
console.log("Expose: replacement done")
}
insertScript(script)
}
request.onerror = function() { console.error("Expose: response was null") }
request.open("get", node.src, true)
request.send()
}
return true
}
Lines 260-267 for easier debugging purposes:
"Object.defineProperty( window.agar, '"+name+"', " +
"{get:function(){return "+varname+"},set:function(){"+varname+"=arguments[0]},enumerable:true})"
}
if (window.top == window.self) {
if (document.readyState !== 'loading')
return console.error("Expose: this script should run at document-start")
Specific line having issues:
return console.error("Expose: this script should run at document-start")
UPDATE:
New issue. Uncaught SyntaxError: Illegal return statement engine.js:282
Lines 281-282 for debugging purposes:
if (!rules)
return console.error("Expose: cant find corresponding rule")
UPDATE 2:
This is my final issue. And this whole thing will be resolved.
It looks like its another return error. But i do not understand how to properly return this part.
Heres the error but its basically the same.
Uncaught SyntaxError: Illegal return statement engine.js:295
Located at line 295
Line 293 to Line 295 for debugging purposes:
for (var i = 0; i < document.head.childNodes.length; i++)
if (tryReplace(document.head.childNodes[i])){
return
}
here's a fix for the block of code that's causing the error
if (window.top == window.self) {
if (document.readyState !== 'loading') {
// don't return
console.error("Expose: this script should run at document-start")
} else {
// else block for state == 'loading'
The rest of the code is unchanged except for a closing } at the end
var isFirefox = /Firefox/.test(navigator.userAgent)
// Stage 1: Find corresponding rule
var rules
for (var i = 0; i < allRules.length; i++)
if (allRules[i].hostname.indexOf(window.location.hostname) !== -1) {
rules = allRules[i]
break
}
if (!rules)
return console.error("Expose: cant find corresponding rule")
// Stage 2: Search for `main_out.js`
if (isFirefox) {
function bse_listener(e) {
tryReplace(e.target, e)
}
window.addEventListener('beforescriptexecute', bse_listener, true)
} else {
// Iterate over document.head child elements and look for `main_out.js`
for (var i = 0; i < document.head.childNodes.length; i++)
if (tryReplace(document.head.childNodes[i]))
return
// If there are no desired element in document.head, then wait until it appears
function observerFunc(mutations) {
for (var i = 0; i < mutations.length; i++) {
var addedNodes = mutations[i].addedNodes
for (var j = 0; j < addedNodes.length; j++)
if (tryReplace(addedNodes[j]))
return observer.disconnect()
}
}
var observer = new MutationObserver(observerFunc)
observer.observe(document.head, {
childList: true
})
}
} // added this closing }
}
The javascript I am using:
javascript: c = '{unit}, ({coords}) {player} |{distance}| {return}';
p = ['Scout', 'LC', 'HC', 'Axe', 'Sword', 'Ram', '***Noble***'];
function V() {
return 1;
}
window.onerror = V;
function Z() {
d = (window.frames.length > 0) ? window.main.document : document;
aid = d.getElementById('editInput').parentNode.innerHTML.match(/id\=(\d+)/)[1];
function J(e) {
vv = e.match(/\d+\|\d+/g);
return (vv ? vv[vv.length - 1].match(/((\d+)\|(\d+))/) : null);
}
function K(e) {
f = parseInt(e, 10);
return (f > 9 ? f : '0' + f);
}
function L(g, e) {
return g.getElementsByTagName(e);
}
function N(g) {
return g.innerHTML;
}
function M(g) {
return N(L(g, 'a')[0]);
}
function O() {
return k.insertRow(E++);
}
function W(f) {
return B.insertCell(f);
}
function P(g, e) {
g.innerHTML = e;
return g;
}
function X(e) {
C = B.appendChild(d.createElement('th'));
return P(C, e);
}
function Y(f) {
return K(f / U) + ':' + K(f % (U) / T) + ':' + K(f % T);
}
U = 3600;
T = 60;
R = 'table';
S = 'width';
s = L(document, R);
for (j = 0; j < s.length; j++) {
s[j].removeAttribute(S);
if (s[j].className == 'main') {
s = L(L(s[j], 'tbody')[0], R);
break;
}
}
D = 0;
for (j = 0; j < s.length; j++) {
s[j].removeAttribute(S);
if (s[j].className = 'vis') {
k = s[j];
if (t = k.rows) {
D = t.length;
break;
}
}
}
for (E = 0; E < D; E++) {
l = t[E];
m = (u = l.cells) ? u.length : 0;
if (m) {
u[m - 1].colSpan = 5 - m;
if (N(u[0]) == 'Arrival:') {
Q = new Date(N(u[1]).replace(/<.*/i, ''));
} else {
if (N(u[0]) == 'Arrival in:') {
v = N(u[1]).match(/\d+/ig);
}
}
if (E == 1) {
G = M(u[2]);
}
if (E == 2) {
w = J(M(u[1]));
}
if (E == 4) {
x = J(M(u[1]));
}
}
}
y = v[0] * U + v[1] * T + v[2] * 1;
n = w[2] - x[2];
o = w[3] - x[3];
F = Math.sqrt(n * n + o * o);
H = F.toFixed(2);
E = D - 2;
s = L(k, 'input');
i = s[1];
h = s[0];
h.size = T;
B = O();
P(W(0), 'Distance:').colSpan = 2;
P(W(1), H + ' Fields').colSpan = 2;
B = O();
X('Unit');
X('Sent');
X('Duration');
X('Name to');
c = c.replace(/\{coords\}/i, w[1]).replace(/\{distance\}/i, H).replace(/\{player\}/i, G);
for (j in p) {
z = Math.round([9.00000000, 10.00000000, 11.00000000, 18.0000000015, 22.00000000, 30.00000000, 35.0000000][j] * T * F);
A = z - y;
if (A > 0) {
I = Y(z);
B = O();
P(W(0), p[j]);
P(W(1), A < T && 'just now' || A < U && Math.floor(A / T) + ' mins ago' || Y(A) + ' ago');
P(W(2), I);
C = W(3);
q = C.appendChild(i.cloneNode(1));
r = C.appendChild(h.cloneNode(1));
r.id = 'I' + j;
r.value = c.replace(/\{duration\}/i, I).replace(/\{sent\}/i, new Date(Q.valueOf() - z * 1000).toLocaleString().replace(/.\d{4}/i, '').replace(/(\w{3})\w*/i, '$1')).replace(/\{return\}/i, new Date(Q.valueOf() + z * 1000).toString().replace(/\w+\s*/i, '').replace(/(\d*:\d*:\d*)(.*)/i, '$1')).replace(/\{unit\}/i, p[j]).replace(/\{attack_id\}/i, aid);
q.onmousedown = new Function('h.value=d.getElementById(\'I' + j + '\').value;');
}
}
}
Z();
The error I receive:
Uncaught TypeError: Cannot read property 'parentNode' of null
A URL looks like this:
game.php?village=2100&id=4348754&type=other&screen=info_command
There are two possibilities:
editInput is a typo, and the actual id of that element is different (ids are case-sensitive).
You are executing this code while the DOM is not ready. To prevent this, execute the code just before the </body> closing tag, or wrap it in an event handler for the load event of window or the DOMContentLoaded event of document.
EDITED How to wrap your code:
window.onload = function() {
//your code here
};
In my case it was a conflict between slider and form validator as they were located on different pages but javascript file would of target both of them simultaneously, so all pages that had no slider would come up with "Cannot read property 'parentNode' of null" error, so I have added to the slider If statement:
if(document.getElementById("mainImage")){
var myImage = document.getElementById("mainImage");
var linkElement = myImage.parentNode;
that solved my problem, try it
I was trying to remove child element from parent:
constructor(private renderer: Renderer2) { }
removeChildEl() {
this.renderer.removeChild(parent, child);
}
but for some reason parent-element does not exist so I need to check via if condition:
removeChildEl() {
if(parent)
this.renderer.removeChild(parent, child);
}
so if the parent exists then remove the child otherwise do nothing.