Is there any shortcut to accomplishing the equivalent of PHP's array_flip function in JavaScript or does it have to be done via brute force looping?
It has to be used for dozens of arrays so even small speedups will probably add up.
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8 Answers
Don't think there's one built in. Example implementation here, though :).
function array_flip( trans )
{
var key, tmp_ar = {};
for ( key in trans )
{
if ( trans.hasOwnProperty( key ) )
{
tmp_ar[trans[key]] = key;
}
}
return tmp_ar;
}
answered Jul 21, 2009 at 13:45
Daniel F. Thornton
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4 Comments
Peter Bailey
@pianoman - I added the hasOwnProperty check. Hope you don't mind =)
Christoph
I wouldn't recommend phpjs.org: the code quality is underwhelming and some functions don't work in IE at all! If you want to program in JS, learn it!
Christoph
also keep in mind that this only works reliably if the property values have a unique string representation
Daniel F. Thornton
@Christoph - See your point, but if the properties aren't unique, one really shouldn't be flipping the array to begin with ಠ_ಠ
ES6 version
const example = { a: 'foo', b: 'bar' };
const flipped = Object.entries(example)
.reduce((obj, [key, value]) => ({ ...obj, [value]: key }), {});
// flipped is {foo: 'a', bar: 'b'}
ES5 version
var example = {a: 'foo', b: 'bar'};
var flipped = Object.keys(example) //get the keys as an array
.reduce(function(obj, key) { //build up new object
obj[example[key]] = key;
return obj;
}, {}); //{} is the starting value of obj
// flipped is {foo: 'a', bar: 'b'}
Comments
Using underscore _.invert
_.invert([1, 2])
//{1: '0', 2: '1'}
_.invert({a: 'b', c: 'd'})
//{b: 'a', d: 'c'}
3 Comments
Con Antonakos
Good solution, but how would you cast the string value, which are the array indices, to type
number? Call another method like map?
Afanasii Kurakin
@ConAntonakos yes, I guess
_.mapObject(_.invert([1, 2]), function (d) {return +d}) will do
Rafał G.
For those using lodash, it has
invert too.with jQuery:
var array_flipped={};
$.each(array_to_flip, function(i, el) {
array_flipped[el]=i;
});
Comments
I guess you are talking about Objects not Arrays
function flip(o){
var newObj = {}
Object.keys(o).forEach((el,i)=>{
newObj[o[el]]=el;
});
return newObj;
}
Otherwise it could be
function flip(o){
var newObj = {}
if (Array.isArray(o)){
o.forEach((el,i)=>{
newObj[el]=i;
});
} else if (typeof o === 'object'){
Object.keys(o).forEach((el,i)=>{
newObj[o[el]]=el;
});
}
return newObj;
}
1 Comment
Domino
The .map function doesn't work at all, it only creates an array of ascending numbers from 0 to length-1 regardless of array content.
The current top answer didn't work as expected for me because key values are offset +1. (and so the returned array tmpArr(0) is also always undefined. So I subtracted 1 from key value and it worked as expected.
function array_flip2 (trans) {
var key
var tmpArr = {}
for (key in trans) {
if (!trans.hasOwnProperty(key)) {
continue
}
tmpArr[trans[parseInt(key)]-1] = (key)
}
return tmpArr
}
Comments
Simple approach
const obj = { a: 'foo', b: 'bar' },
keys = Object.keys(obj),
values = Object.values(obj),
flipped = {};
for(let i=0; i<keys.length; i++){
flipped[values[i]] = keys[i];
}
console.log(flipped);//{foo: "a", bar: "b"}
Comments
I might flip keys and values this way:
let arr = "abc".split('');
let flip = []
arr.forEach((n, i) => {
flip[Object.values(arr)[i]] = Object.keys(arr)[i];
})
console.log(Object.keys(arr), Object.values(arr));
console.log(Object.keys(flip), Object.values(flip));Depending on how large your initial array is you may want to store the keys/values in their own respective arrays.
