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Array

  • Shallow copy
  • Deep copy

Don't use for..in in Array as it is an Object.

Splice

Create a new array from existing array.

// Take
// Start from position=3 and take 3 elements
var a = ['a', 'b', 'c', 'd', 'e', 'f', 'g'];
a.splice(3, 3); // d,e,f

// Insertion
// Specifying 0 means nothing to remove
splice(7, 0, newElement);

Slice

To make arguments a real array:

Array.prototype.slice.call(arguments)
[].slice.call(arguments)

var toString = {}.toString;
var hasOwn = {}.hasOwnProperty;

// copy the data
var data = this.state.data.slice();
Array.from(this.state.data) // Same as slice but in ES6

Sort

sort() works very well with strings, but not so well with numbers as it sort lexicographically. sort() also modify the array which may not be good for React.

// The ordering function to sort numerically rather than lexicographically
function compare(a, b) {
  return a - b;
}

a.sort(compare);
// Sort column from a <table>
var column = e.target.cellIndex;
var data = this.state.data.slice();
data.sort(function(a, b) {
  return a[column] > b[column];
});
this.setState({
  data: data
});

reduce

reduce() applies a function to an accumulator and successive elements until the end of the array is reached, yielding a single value.

There is also the reverse reduceRight().

map

Immutable as it returns a new array. However, filter is mutable!

function first(word) {
  return word[0];
}

var words = ['for', 'your', 'information'];
var acronym = words.map(first).join('');
// Weird - WTF!
// map actually iterate over values instead of using a regular for loop like join
// map and join have different semantic!

[undefined, undefined, undefined,].join("+"); // "++"

[undefined, undefined, undefined,].map(x => "+"); // ["+", "+", "+"]

[,,,].join("+"); // "++"

[,,,].map(x => "+"); // Firefox: [ <3 empty slotes> ]

every and some