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If I have this JavaScript

var someVar = someOtherVar == null;

What values of someOtherVar will cause someVar to be true?

Note: From what I have read, if someOtherVar is undefined then someVar will be true. But I am changing a lot of my code to use this style of comparison, and I want to be sure that nothing else will result in a truthy statement (besides null and undefined).

Vaccano
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  • From my understanding you are correct in saying that undefined and null are the only two values that will result in a truthy value here. – Ken Apr 24 '17 at 20:28
  • Since most linters will not like `someOtherVar == null`, why not go the explicit route and not have to worry about "other values"? `var someVar = (someOtherVar === null || someOtherVar === undefined)` – gforce301 Apr 24 '17 at 20:32
  • @gforce301 - because I am using typescript 2.0 and for some reason beyond my understanding, I can't do a compare to undefined anymore... ( see http://stackoverflow.com/questions/43594261/is-there-a-consistent-way-to-test-for-undefined-and-null-in-typescript-2-0 for details) – Vaccano Apr 24 '17 at 20:36
  • @Vaccano I see. I have had to do this before: `var undefined;` and then you can do `if (callerParam != undefined || callerParam != null)` – gforce301 Apr 24 '17 at 20:39
  • Doesn't the [duplicate](http://stackoverflow.com/questions/28975896/is-there-a-dedicated-function-to-check-null-and-undefined-in-typescript) given for your previous question answer this? I don't understand why you're asking basically the same thing again. –  Apr 24 '17 at 20:48

2 Answers2

5

What values of someOtherVar will cause someVar to be true?

Only null and undefined.

The algorithm in the spec is relatively straight forward:

  1. If Type(x) is the same as Type(y), then Return the result of performing Strict Equality Comparison x === y.
  2. If x is null and y is undefined, return true.
  3. If x is undefined and y is null, return true.
  4. If Type(x) is Number and Type(y) is String, return the result of the comparison x == ToNumber(y).
  5. If Type(x) is String and Type(y) is Number, return the result of the comparison ToNumber(x) == y.
  6. If Type(x) is Boolean, return the result of the comparison ToNumber(x) == y.
  7. If Type(y) is Boolean, return the result of the comparison x == ToNumber(y).
  8. If Type(x) is either String, Number, or Symbol and Type(y) is Object, return the result of the comparison x == ToPrimitive(y).
  9. If Type(x) is Object and Type(y) is either String, Number, or Symbol, return the result of the comparison ToPrimitive(x) == y.
  10. Return false.

Step 1 handles the case null == null. Step 2 and 3 handle the cases null == undefined and undefined == null. Step 6 and 7 handle the cases where the other value is a boolean, so then you end up comparing a number against null. Since null is not an object, number, string, boolean or symbol, non of the other steps apply, so false is returned (step 10).

What coerces from a null compare in JavaScript?

Only if the other value is a boolean it would be coerced to a number, so you end up comparing either 0 == null or 1 == null, both of which are false.

In all other case, no type coercion is happening (nothing is coerced to null and null is not coerced to any other data type).

Felix Kling
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1

Looks like only null and undefined indeed: 

const arr = ['string', true, false, 0, null, undefined, NaN, {}, [], 4, -1, -0];

const r = arr.map((e,i) => {
    return { i, e, bool: e == null }
}).filter(e => e.bool);

// return elements which result in 'null'
// i: index inside initial array 'arr'

console.log(JSON.stringify(r, null, 2));
Egor Stambakio
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