For an interview this week, I was doing an online technical interview and under pressure, I quickly used a check to see if a value existed.
if (x) { numberArray.push(x); }; However, the data I was filtering contained values of 0 and null that needed to be treated differently and intentionally undeclared values that I would usually catch in my own code base or with a linter.
The rules I was trying to filter to could be summarized a such:
const v = 'abc' // exists const w = 0 // exists const x = null // nope const y = undefined // nope // const z // nope My first attempt was equivalent to:
if (x == true) { //Do stuff } 'x' is cast to its boolean equivalent type, but 0 in javascript is the only number that casts to false.
The second area where it deviated is for cases like 'z' where a variable is unassigned or undefined and a comparison is being made, JavaScript will error out. This is not a great user experience.
Here are the results of a few of my tests until I eventually arrived at a way of checking a values existence that I needed.
Tests
The quick and dirty with some pitfalls
console.log(v ? 'exists': 'nope'); // exists console.log(w ? 'exists': 'nope'); // nope console.log(x ? 'exists': 'nope'); // nope console.log(y ? 'exists': 'nope'); // nope console.log(z ? 'exists': 'nope'); // ReferenceError: z is not defined Captures 0, but now also null values and still errors out
console.log(v !== undefined ? 'exists': 'nope'); // exists console.log(w !== undefined ? 'exists': 'nope'); // exists console.log(x !== undefined ? 'exists': 'nope'); // exists console.log(y !== undefined ? 'exists': 'nope'); // nope console.log(z !== undefined ? 'exists': 'nope'); // ReferenceError: z is not defined Closer, but now we are casting types again, and still doing the comparison that breaks on undeclared variables
console.log(v != null ? 'exists': 'nope'); // exists console.log(w != null ? 'exists': 'nope'); // exists console.log(x != null ? 'exists': 'nope'); // nope console.log(y != null ? 'exists': 'nope'); // nope console.log(z != null ? 'exists': 'nope'); // ReferenceError: z is not defined The recommended safe way for most applications
console.log(typeof v !== 'undefined' ? 'exists': 'nope'); // exists console.log(typeof w !== 'undefined' ? 'exists': 'nope'); // exists console.log(typeof x !== 'undefined' ? 'exists': 'nope'); // exists console.log(typeof y !== 'undefined' ? 'exists': 'nope'); // nope console.log(typeof z !== 'undefined' ? 'exists': 'nope'); // nope Extended version of the recommended method for the filter I needed
console.log(typeof v !== 'undefined' && v !== null ? 'exists': 'nope'); // exists console.log(typeof w !== 'undefined' && w !== null ? 'exists': 'nope'); // exists console.log(typeof x !== 'undefined' && x !== null ? 'exists': 'nope'); // nope console.log(typeof y !== 'undefined' && y !== null ? 'exists': 'nope'); // nope console.log(typeof z !== 'undefined' && z !== null ? 'exists': 'nope'); // nope TLDR:
use (typeof value !== 'undefined')
if (typeof value !== 'undefined') { //Do stuff } Bonus:
While every number but 0 casts to true, booleans cast to a number are:
true -> 1
false -> 0
This leads to fun cases like:
-1 == true; // false -1 == false // false -1 ? true : false; // true This is because after casting to types the equivalent expressions are:
-1 == 1; // false -1 == 0; // false true ? true : false; // true 
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