Question 236433 in Javascript Question posted in
A very good W3school tutorial can be found here.

In Javascript, Why do we have to use the undefined keyword?

I have a hashmap, and i’m doing a check to see if a key is not undefined. However, my code only works when I specify that. For instance:

if (map[key] !== undefined) {
   /* code will run */
}

This code works as expected. However, I wanted to shorten the code to just:

if (map[key]) {
   /* code will not run */
}

Shouldn’t these two be the same thing? The code will still run but I only get the expected output for the first code snippet.

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2

Answers


  1. 0

    When you check if (map[key]) that is saying "does this value appear to be falsey". Any value can be passed to an if statement, so what happens when you pass a string 'foo' – will it be true or false?

    Lots of other value types can evaluate to a boolean-like value, which is why you want to check for the specific value with map[key] !== undefined in some cases, and in others you might not. It just depends.

    Run this demo to see how the different value types are treated here

    var arr = [0, 1, '', 'a', true, false, null, undefined];

arr.forEach((val) => {
  console.log(`${val} is a ${typeof val}`);
  
  if (val) {
    console.log(' - truthy');
  }

  if (val !== undefined) {
     console.log(' - not undefined');
  }
});
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  2. 0

    I’m not sure what you mean by "will not run". If the key you’re accessing has a boolean value of false it will NOT run.
    ie

    const map = {};
map['something'] = false;

    if (map[‘something’]) will not run (regardless if the key is there or not).

    Note map[‘something’] could have been false, NaN, ”, undefined, 0,
    all which would return false.

    (() => {

// when using Map object.
const map = new Map();
map.set(4, true);

// you get the value by using .get() not [].
console.log(map.get(4));
// true

// when using object literal
const map2 = {};
// you can access the key using []
map2[4] = true;
console.log(map2[4]);
// true
console.log(map2[4] === true ? true : false);
// true

})();

    A more accurate way to determining if a key exists is to use hasOwnProperty. Do NOT use in as it includes prototype properties as well.

    Example:
    if (map.hasOwnProperty(THE_SEARCHED_FOR_KEY)){ … }

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