JavaScript predicate

last modified October 18, 2023

In this article we show how to use predicates in JavaScript.

A predicate in general meaning is a statement about something that is either true or false. In programming, predicates represent single argument functions that return a boolean value.

JS predicate with find

The find function takes a predicate to determine the first element that satisfies it.

function isPositive(e) {
    return e > 0;
}

let vals = [-2, -3, 0, 4, 3, -1, 1, 7];

let res = vals.find(isPositive);
console.log(res);

let res2 = vals.find(e => e > 0);
console.log(res2);

In the example, we find the first positive value in the array.

function isPositive(e) {
    return e > 0;
}

The first predicate is a classic function definition.

let res2 = vals.find(e => e > 0);

The second predicate is an annonymous arrow function.

$ node main.js 
4
4

JS predicate with filter

The filter function creates a new array filled with elements that satisfy the provided predicate.

let vals = [-2, -3, 0, 4, 3, -1, 1, 7];

let pos = vals.filter(e => e > 0);
console.log(pos);

let neg = vals.filter(e => e < 0);
console.log(neg);

let evs = vals.filter(e => e % 2 === 0);
console.log(evs);

In the example, we filter out positive values, negative values, and even values.

$ node main.js 
[ 4, 3, 1, 7 ]
[ -2, -3, -1 ]
[ -2, 0, 4 ]

JS predicate with every/some

The every function determines whether all the members of an array satisfy the specified predicate. The some function determines whether any element of the array satisfies the predicate.

let vals = [-2, -3, 0, 4, 3, -1, 1, 7];

if (vals.every(e => e > 0)) {
    console.log('all values are positive');
} else {
    console.log('not all values are positive');
}

if (vals.some(e => e > 0)) {
    console.log('at least one value is positive');
} else {
    console.log('no value is positive');
}

In the example we check, if all values of the array are positive and if at least one value is positive.

$ node main.js 
not all values are positive
at least one value is positive

JS negating predicates

In the next example, we define a negate function which negates the given predicate.

function negate(other) {
    return e => { return !other(e) };
};

let vals = [-2, -3, 0, 4, 3, -1, 1, 7];

let res = vals.filter(negate(e => e > 0));
console.log(res);

let res2 = vals.filter(negate(negate(e => e > 0)));
console.log(res2);

We use the negate function with filter.

function negate(other) {
    return e => { return !other(e) };
};

In the negate function, we use the ! operator, which computes logical negation of its operand.

let res = vals.filter(negate(e => e > 0));

We negate the output of the arrow function, which returns positive values.

let res2 = vals.filter(negate(negate(e => e > 0)));

We apply the negate funtion twice.

$ node main.js 
[ -2, -3, 0, -1 ]
[ 4, 3, 1, 7 ]

Source

JavaScript - language reference

In this article we have worked with predicates in JavaScript.

Author

My name is Jan Bodnar and I am a passionate programmer with many years of programming experience. I have been writing programming articles since 2007. So far, I have written over 1400 articles and 8 e-books. I have over eight years of experience in teaching programming.

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