This method just returns the first argument.

var res1 = _.identity(10, 20);
// res1 now is 10

var res2 = _.identity("hello", "world");
// res2 now is "hello"

What does _.identity mean in lodash documentation?

This method is used throughout lodash documentation instead of function(x){return x;} (or ES6 equivalent x => x).

It usually means either "no transformation" or when used as a predicate: the truthiness of the value.

Example of _.identity in documentation of _.times

The _.times function takes two arguments. Its expressed like this in the documentation: var res = _.times(n, [iteratee=_.identity])

The iteratee is used to transform the values as they are iterated over.

The documentation shows that the iteratee parameter is optional, and if it is omitted it will have the default value of _.identity, which in this case means "no transformation"

var res = _.times(5);      // returns [0, 1, 2, 3, 4]

// means the same as:
var res = _.times(5, _.identity);

// which again does the same as:
var res = _.times(5, function(x){ return x; });

// or with the ES6 arrow syntax:
var res = _.times(5, x => x);

Example of _.identity in documentation of _.findKey and _.findLastKey

The _.findKey and _.findLastKey functions takes two arguments. Its expressed like this in the documentation: _.findKey(object, [predicate=_.identity]) and _.findLastKey(object, [predicate=_.identity])

This again means that the second parameter is optional, and if it is omitted it will have the default value of _.identity, which in this case means the first (or last) of "anything that is truthy"

var p = {
    name: "Peter Pan",
    age: "Not yet a grownup",
    location: "Neverland"
};

var res1 = _.findKey(p);        // returns "name"
var res2 = _.findLastKey(p);    // returns "location"

// means the same as:
var res1 = _.findKey(p, _.identity);
var res2 = _.findLastKey(p, _.identity);

// which again means the same as:
var res1 = _.findKey(p, function(x) { return x; }
var res2 = _.findLastKey(p, function(x) { return x; }

// or with ES6 arrow syntax:
var res1 = _.findKey(p, x => x);
var res2 = _.findKey(p, x => x);