Tips for golfing in JavaScript

Question

What general tips do you have for golfing in JavaScript? I'm looking for ideas that can be applied to code golf problems in general that are at least somewhat specific to JavaScript (e.g. "remove comments" is not an answer).

Note: Also see Tips for Golfing in ECMAScript 6 and above

Answer 1

If returning void, see if it is shorter to return something useful. This is kinda language-agnostic here.

This isn't an easy apply-anywhere thing, either, though. Word of warning, make sure your parentheses are balanced.

In ES6, in this example, 6 bytes saved

let l=x=>console.log(m+x),x=1,a;l(a=f(x));l(a=g(a));l(h(a))
let l=x=>(console.log(m+x),x),x=1;l(h(l(g(l(f(x))))))

In ES5, it is only 1 byte saved

function l(x){console.log(m+x)}var x=1,a;l(a=f(x));l(a=g(a));l(h(a))
function l(x){console.log(m+x);return x}var x=1;l(h(l(g(l(f(x))))))

Answer 2

Convert ints to strings by adding an empty string

For example:

39323+""

Returns:

"39323"

Update:

Adding [] works too

Answer 3

Adding Values with Implicit Casting

Improved zzzzBov solution:

//not so good
-(-a-b)==c;

//best
a- -b==c;
a-+-b==c;

We save 2 characters by using these solutions.

Answer 4

Use Bitwise as Logic Operators When Dealing With Booleans

a = 1 //although this would usually be a boolean expression
b = 0 //same
if(a&&b)c()
if(a&b)c()

Then, use && lazy evaluation to make a chain of ampersands:

a&b&&c()
if(a&&b)c()

Saves 3 characters

Answer 5

Function

If you need a function in as few bytes as possible, and any function will do (perhaps you just want to access some of the goodies from Function.prototype), then here are some options (starting with large ones):

Function.prototype
[].map
Date
CSS     (available in modern browsers)
Map     (ES6: available in Node and modern browsers)
Set     (ES6: available in Node and modern browsers)
URL     (available in very old browsers, but not in Node)

So if you want a reference to the call function, you can get it like this:

c=URL.call

Answer 6

If iterating through own properties, prefer Object.keys.

15 bytes saved

for(let p in o)if(o.hasOwnProperty(e)){/* ... */}
for(let p of Object.keys(o)){/* ... */}
Object.keys(o).map(p=>{/* ... */})

This is also the case for ES5, where it is 7 bytes saved.

for(var p in o)if(o.hasOwnProperty(e)){/* ... */}
Object.keys(o).map(function(p){/* ... */})

If you do that more than once, alias it as a function.

In this ES6 example, 6 bytes saved. It still saves bytes in ES5, but only if used 3 times or more.

Object.keys(o).map(p=>{/* ... */})Object.keys(o).map(p=>{/* ... */})
i=f=>Object.keys(o).map(f);i(p=>{/* ... */});i(p=>{/* ... */})

Answer 7

ES6-specific: avoid Function#bind()

Self-explanatory, 7 bytes saved

f.bind(null,x,...xs)
_=>f(x,...xs)

Use sloppy mode to avoid variable declarations

Here, 8 bytes saved

a.forEach(e=>{let b=e+1,c=d(b)+2;f(e);g(b);h(c);i(b,c)})
a.forEach(e=>{f(e);g(b=e+1);h(c=d(b)+2);i(b,c)})

// Even better, reuse variables and use `Array#map()`
// Drops an additional 6 bytes
a.map(e=>{f(e++);g(e);h(b=d(e)+2);i(e,b)})

This also holds in ES5. Here, 8 bytes saved

a.forEach(function(e){let b=e+1,c=d(b)+2;f(e);g(b);h(c);i(b,c)})
a.forEach(function(e){f(e);g(b=e+1);h(c=d(b)+2);i(b,c)})

// Even better, reuse variables and use `Array#map()`
a.map(function(e){f(e++);g(e);h(b=d(e)+2);i(e,b)})

Answer 8

If you are ever in a situation where you have an array a and you need to get its last element as an array of itself, use Array#slice():

[a[a.length-1]]    // 15
a.slice(-1)        // 11

Answer 9

This is one of my favorites - ES6

'da,dad,sa'.split``
 ["d", "a", ",", "d", "a", "d", ",", "s", "a"]

'da,dad,sa'.split`,`
["da", "dad", "sa"]

Answer 10

If you are using the same function more than once, it's often useful to reference the function to a new function name. I.e:

Instead of:

Math.sqrt(4)
Math.sqrt(16)
Math.sqrt(100)

you can do:

r=Math.sqrt
r(4)
r(16)
r(100)

Answer 11

Use the simplest shortening method available - your variable declaration!

var myName = "Jack";

Obviously is very long compared to:

m="Jack"

It's a whole 12 characters shorter. You have all 64 of these single-character variable names available:

abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ$_

And also - never use spaces or semicolons in your variable naming - changing m = "Mum"; to m="Mum" saves another three characters.

Answer 12

Square root hack

Use (x+2)**(1/2) instead of Math.sqrt(x+2).

Answer 13

Converting an array of strings into numbers

Take the array ["32", "0x30", "0o10", "7.642", "1e3"]. The simple way to convert this to numbers would be .map(n=>+n) or .map(Number).

However, assuming you know everything is a valid number, you can simply use .map(eval), saving at least a byte.

Answer 14

Array#concat() and the spread operator

This largely depends on the situation.

---

Combining multiple arrays.

Prefer the concat function unless cloning.

0 bytes saved

a.concat(b)
[...a,...b]

3 bytes wasted

a.concat(b,c)
[...a,...b,...c]

3 bytes saved

a.concat()
[...a]

6 bytes saved

// Concatenate array of arrays
[].concat.apply([],l)
[].concat(...l)

---

Prefer using an already existing array to Array#concat().

Easy 4 bytes saved

[].concat(a,b)
a.concat(b)

Answer 15

When stringifying dates, .toJSON saves 5 bytes over .toISOString. Apparently this was supported as far back as Firefox 4, but this answer is only the sixth on PPCG to mention it.

Answer 16

Shortening Promise Chains with async/await

Sometimes you can shorten longer promise chains with async/await. The main benefit is from getting rid of the beginning of the arrow function in each then callback. .then(x=>x (10) gets replaced with await( (-4), but you first pay with async (+6). So to make up for the initial overhead of 6 bytes, you'd need at least two then chains to get any benefit.

+-------------+----------------+
| then chains | async overhead |
+-------------+----------------+
| 0           | +6             |
| 1           | +2             |
| 2           | -2             |
| 3           | -4             |
| …           | …              |
+-------------+----------------+

Example 1

x=>x().then(y=>y.foo()).then(z=>z.bar())
async x=>await(await(x()).foo()).bar()

Example 2

u=>fetch(u).then(r=>r.text()).then(t=>/\0/.test(t))
async u=>/\0/.test(await(await fetch(u)).text()))

Answer 17

Abusing template literals

You can often save two bytes on parentheses by replacing

a.join('+')

with

a.join`+`

This works for most properties that take in a single string.

This has been used in other answers, but there isn't an answer dedicated to this specifically.

Answer 18

If you need to do something different depending on several different values for a string, instead of:

if(v=='hello'){
  j=1
}else if(v=='goodbye'){
  j=2
//...

consider:

j={hello:1,goodbye:2,...}[v]

Answer 19

You can check if a value is *truish by simply passing it:

if(val){...}

*everything different than 0, 0n, "", false, null, undefined and NaN is evaluated to true !

This method can be applied with many other functions and operators:

• ternary operator val?"true":"false";
• for loop for(;val;){...}
• while loop while(val){...}
• etc...

Answer 20

Remove duplicates from array

a.filter(e=>!(t[e]=e in t)) 

let unique= (a,t={})=> a.filter(e=>!(t[e]=e in t));

// "stand-alone" version working with global t:
// a1.filter((t={},e=>!(t[e]=e in t)));

// Test data
let a1 = [5,6,0,4,9,2,3,5,0,3,4,1,5,4,9];
let a2 = [[2, 17], [2, 17], [2, 17], [1, 12], [5, 9], [1, 12], [6, 2], [1, 12]];
let a3 = ['Mike', 'Adam','Matt', 'Nancy', 'Adam', 'Jenny', 'Nancy', 'Carl'];

// Results
console.log(JSON.stringify( unique(a1) ))
console.log(JSON.stringify( unique(a2) ))
console.log(JSON.stringify( unique(a3) ))

O(n) performance; we assume your array is in a and t={}. Explanation here

And shorter but slower version (which not work with 2D arrays)

[...new Set(a)]

let unique = a => [...new Set(a)];

// Test data
let a1 = [5, 6, 0, 4, 9, 2, 3, 5, 0, 3, 4, 1, 5, 4, 9];
let a3 = ['Mike', 'Adam', 'Matt', 'Nancy', 'Adam', 'Jenny', 'Nancy', 'Carl'];

// Results
console.log(JSON.stringify(unique(a1)))
console.log(JSON.stringify(unique(a3)))

Answer 21

Add elements to Array

The concat is more useful than push in many situations because it returns whole array e.g. we want from d=[{a:1,b:2,c:3},{a:7,b:8,c:9}] generate 'pivot object' p={"a":[1,7],"b":[2,8],"c":[3,9]}

let p={}, d=[{a:1,b:2,c:3},{a:7,b:8,c:9}]; 

d.map(x=> Object.keys(x).map(k=> p[k]= (p[k]||[]).concat(x[k]) ))

console.log(JSON.stringify(p));

Answer 22

The 9**999 is Infinity shortcut (2 characters shorter)

console.log( 9**999===Infinity )

Answer 23

Converting a string to an int/float by subtracting a empty array

Before(22 bytes)

parseFloat("12.52463")

After(13 bytes, saved 9 bytes)

"12.52463"-[]