How to avoid assignments while comparing in Javascript? [closed]

Answer 1

Use if (a === b) { do stuff ; } that's three equals, ===

In Javascript you often need three equals, ===, as this requires primitives to have both the same type and value, and will not perform type coercion which, although you don't mention it, can be another source of confusion.

If you leave off an equals, it becomes two equals comparison, a == b, which might not work quite as expected if you are a beginner or unaware of the quirks of this comparison in Javascript, but at least is not an assignment.

Answer 2

Edit: Didn't see the questioners requirement to avoid this method.

Use strict comparison operator (===). You can also change the order of the values being compared which will fail if you use assignment statement rather than comparison operator.

Fails:

var test = 1;
if(2 = test) {
 console.log(test);
}

The above code will throw an exception since you can't assign a value to the constant value.

Works:

var test = 1;
if(2 == test) {
 console.log(test);
}

Answer 3

The assignment operator (=) in JavaScript evaluates to the right-side value of the expression, which means that test = 2 evaluates to 2 and since all values that aren't falsey (0, false, null, undefined, '', NaN) are thruthy by definition, the if condition was entered.

A good practice would be to always use the === sign when checking for equality and !== for inequality and there are tools that will help you enforcing those rules, like JSHint

Answer 4

a = b is an assignment. Meaning a will equal to b.

a == b is a comparison. Meaning does a equal to b.

a === b is the same as == however no type conversion is done. Meaning does a equal to b, but also is it the same object/type?

Where your going wrong with your code is

var test = 1;
if(test = 2) { //this is true, because test now equals 2.
   console.log(test);
}

Answer 5

Lint it. There are tools that can automatically analyze your Javascript and catch common errors like this.

Answer 6

The answer in JavaScript is pretty easy: always use strict comparison. This will avoid not only assignment errors, but also some non-intuitive behavior that can come from JavaScript's weak comparison rules.