What is context in _.each(list, iterator, [context])?

Question

I am new to underscore.js. What is the purpose of [context] in _.each()? How should it be used?

Answer 1

The context lets you provide arguments at call-time, allowing easy customization of generic pre-built helper functions.

some examples:

// stock footage:
function addTo(x){ "use strict"; return x + this; }
function pluck(x){ "use strict"; return x[this]; }
function lt(x){ "use strict"; return x < this; }

// production:
var r = [1,2,3,4,5,6,7,8,9];
var words = "a man a plan a canal panama".split(" ");

// filtering numbers:
_.filter(r, lt, 5); // elements less than 5
_.filter(r, lt, 3); // elements less than 3

// add 100 to the elements:
_.map(r, addTo, 100);

// encode eggy peggy:
_.map(words, addTo, "egg").join(" ");

// get length of words:
_.map(words, pluck, "length"); 

// find words starting with "e" or sooner:
_.filter(words, lt, "e"); 

// find all words with 3 or more chars:
_.filter(words, pluck, 2); 

Even from the limited examples, you can see how powerful an "extra argument" can be for creating re-usable code. Instead of making a different callback function for each situation, you can usually adapt a low-level helper. The goal is to have your custom logic bundling a verb and two nouns, with minimal boilerplate.

Admittedly, arrow functions have eliminated a lot of the "code golf" advantages of generic pure functions, but the semantic and consistency advantages remain.

I always add "use strict" to helpers to provide native [].map() compatibility when passing primitives. Otherwise, they are coerced into objects, which usually still works, but it's faster and safer to be type-specific.