如何使用 lodash 中的 includes 方法检查对象是否在集合中？

Question

lodash lets me check for membership of basic data types with includes :

_.includes([1, 2, 3], 2)
> true

But the following doesn't work:

_.includes([{"a": 1}, {"b": 2}], {"b": 2})
> false

This confuses me because the following methods that search through a collection seem to do just fine:

_.where([{"a": 1}, {"b": 2}], {"b": 2})
> {"b": 2}
_.find([{"a": 1}, {"b": 2}], {"b": 2})
> {"b": 2}

What am I doing wrong? How do I check for the membership of an object in a collection with includes ?

edit: question was originally for for lodash version 2.4.1, updated for lodash 4.0.0

Answer 1

The includes (formerly called contains and include ) method compares objects by reference (or more precisely, with === ). Because the two object literals of {"b": 2} in your example represent different instances, they are not equal. Notice:

({"b": 2} === {"b": 2})
> false

However, this will work because there is only one instance of {"b": 2} :

var a = {"a": 1}, b = {"b": 2};
_.includes([a, b], b);
> true

On the other hand, the where (deprecated in v4) and find methods compare objects by their properties, so they don't require reference equality. As an alternative to includes , you might want to try some (also aliased as any ):

_.some([{"a": 1}, {"b": 2}], {"b": 2})
> true

Answer 2

Supplementing the answer by pswg , here are three other ways of achieving this using lodash 4.17.5 , without using _.includes() :

Say you want to add object entry to an array of objects numbers , only if entry does not exist already.

let numbers = [
    { to: 1, from: 2 },
    { to: 3, from: 4 },
    { to: 5, from: 6 },
    { to: 7, from: 8 },
    { to: 1, from: 2 } // intentionally added duplicate
];

let entry = { to: 1, from: 2 };

/* 
 * 1. This will return the *index of the first* element that matches:
 */
_.findIndex(numbers, (o) => { return _.isMatch(o, entry) });
// output: 0


/* 
 * 2. This will return the entry that matches. Even if the entry exists
 *    multiple time, it is only returned once.
 */
_.find(numbers, (o) => { return _.isMatch(o, entry) });
// output: {to: 1, from: 2}


/* 
 * 3. This will return an array of objects containing all the matches.
 *    If an entry exists multiple times, if is returned multiple times.
 */
_.filter(numbers, _.matches(entry));
// output: [{to: 1, from: 2}, {to: 1, from: 2}]

If you want to return a Boolean , in the first case, you can check the index that is being returned:

_.findIndex(numbers, (o) => { return _.isMatch(o, entry) }) > -1;
// output: true

Answer 3

You could use find to solve your problem

https://lodash.com/docs/#find

const data = [{"a": 1}, {"b": 2}]
const item = {"b": 2}


find(data, item)
// > true