ECMA6 is changing our coding style

The in operator returns true if the specified property is in the specified object or its prototype chain.

var car = {make: ‘Honda’, model: ‘Accord’, year: 1998};
console.log(‘make’ in car);
//expected output: true

Don’t forget, arrays are objects too — which means we can also use the loop on Arrays:

const arr = ['cat', 'dog', 'fish'];
for (let i in arr)
}// cat
// dog
// fish
var names = ['Alice', 'Bob', 'Tiff', 'Bruce', 'Alice'];

var countedNames = names.reduce(function (allNames, name) {
if (name in allNames) {
else {
allNames[name] = 1;
return allNames;
}, {});

// countedNames is:
// { 'Alice': 2, 'Bob': 1, 'Tiff': 1, 'Bruce': 1 }

+Tries to convert the operand into a number
- Tries to convert the operand into a number and negates after

Arrays do not form a separate language type. They are based on objects.So typeof does not help to distinguish a plain object from an array:

alert(typeof {}); // object
alert(typeof []); // same

But arrays are used so often that there’s a special method for that: Array.isArray(value). It returns true if the value is an array, and false otherwise.

alert(Array.isArray({})); // false
alert(Array.isArray([])); // true

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