This question already has answers here:
Javascript if in x [duplicate]
(3 answers)
Closed 6 years ago.
Hi i'm following a tutorial learning to use javascript.
function move(keyclick) {
if(40 in keyclick) {}
playerMove.x++;
render(); }
What does the 'in' word mean? I understand what the function is doing, but why not just use ==
?
Thanks
The in operator is true if the string on the LHS is the name of a property that exists on the object on the RHS.
== tests if a value matches another value, which is entirely different.
The in operator returns true if the specified property is in the specified object (cited from https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Operators/in).
Related
This question already has answers here:
Determine whether an array contains a value [duplicate]
(18 answers)
What is the difference between "in operator" and "includes()" for JavaScript arrays
(4 answers)
Closed 1 year ago.
I need to execute some code. But my code is not working properly. I have made an simple example to demonstrate the malfunction.
When I run
console.log("val" in Object.values({key:"val"})); //returns false
It gives me false. But if I run
console.log(Object.values({key:"val"}))
outputs => ['val']
I don't understand if it is supposed to work like this. If yes. Why ?
Thanks in Advance.....:)
MDN says "the in operator returns true if the specified property is in the specified object or its prototype chain.", Object.values returns an array. To check if an item exists in an array, use the Array.includes method.
console.log(
Object.values({key:"val"}).includes("val")
); // Returns true
This question already has answers here:
Why can't I access a property of an integer with a single dot?
(5 answers)
Closed 4 years ago.
I guess that javascript will parse (2).valueOf() to new Number(2).valueOf() but why it doesn't for the first one ?
According to the operator precedence, the grouping operator shall have a higher priority than the member access https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Operators/Operator_Precedence
So why (2) is not be evaluated first and yield 2 instead of be parsed to new Number(2) ?
Because in 2.valueOf the . is considered to be as a part of 2 instead of being understood as method accessing.
That is why 2..valueOf() works.
console.log(2..valueOf());
This question already has answers here:
What does the construct x = x || y mean?
(12 answers)
How does javascript logical assignment work?
(6 answers)
JavaScript OR (||) variable assignment explanation
(12 answers)
Closed 4 years ago.
I just saw a line of code like this one below and I was intrigued by the use of ||
const myCars = ['BMW','Audi','VW']
const foo = myCars.length || [];
Does this mean if myCars.length was to ever be undefined that foo would equal an empty array instead of undefined?
Yes, how it should be read is:
if 'myCars' doesn't have a length (e.g. no values), the constant foo should be set to [].
Note: https://www.w3schools.com/jsref/jsref_length_array.asp, specifically the return value of the .length: "A Number, representing the number of elements in the array object".
this is Short-circuit evaluation in Javascript. its Unique in JS to USE || operator because other languages use this operator in conditional statements only. please read this
https://en.wikipedia.org/wiki/Short-circuit_evaluation
This question already has answers here:
In Typescript, what is the ! (exclamation mark / bang) operator when dereferencing a member?
(5 answers)
Closed 5 years ago.
I've seen in some React/TypeScript implementations such as :
ref={ ref => this.container = ref! }
What does the exclamation point means in ref!?
Is that something specific in TypeScript, or a new standard JavaScript notation?
In TypeScript, a postfix ! removes null and undefined from the type of an expression.
This is useful when you know, for reasons outside TypeScript's inference ability, that a variable that "could" be null or undefined actually isn't.
This question already has answers here:
Best way to find if an item is in a JavaScript array? [duplicate]
(8 answers)
How do I check if an array includes a value in JavaScript?
(60 answers)
Closed 6 years ago.
Python programmer here.
I don't know how to write this. I tried using 'if !in' and '!if in', but I don't know how. Tried to Google it but got no results.
The correct syntax is
if(!condition){
expression();
}
Note that you need parenthesis around the condition.
#plalx wants a formal definition, and here you go:
IfStatement:
if(Expression) Statement else Statement
if(Expression) Statement
In case of any ambiguity the else would be matched with the nearest if.
If you have some value:
var excludeMe = "something unwanted";
Then you can use the following if statement:
if(myTestCase !== excludeMe) { //do something...}
Keep in mind that != does not check type and !== does check type. So, 1 != "1" is false and 1 !== "1" is true.