What's the equivalent of "has_key" in javascript?

hasOwnProperty:

if(Object.prototype.hasOwnProperty.call(dictionary, key)) {
    // ...

You can also use the in operator, but sometimes it gives undesirable results:

console.log('watch' in dictionary); // always true

Either with the in operator:

if('school' in dictionary) { …

Or probably supported in more browsers: hasOwnProperty

if({}.hasOwnProperty.call(dictionary, 'school')) { …

Could be problematic in border cases: typeof

if(typeof(dictionary.school) !== 'undefined') { …

One must not use != undefined as undefined is not a keyword:

~~if(dictionary.school != undefined) { …~~

But you can use != null instead, which is true for null, undefined and absent values:

if(dictionary.school != null) { …

The 'in' operator.

if ('school' in dictionary)

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