Mastering JavaScript Functional Programming - Second Edition

Higher-level predicates – some, every

The last functions we are going to consider greatly simplify going through arrays to test for conditions. These functions are as follows:

• every(), which is true if and only if every element in the array satisfies a given predicate
• some(), which is true if at least one element in the array satisfies the predicate

For example, we could easily check our hypothesis about all the countries having negative coordinates:

markers.every(v => v.lat < 0 && v.lon < 0); // falsemarkers.some(v => v.lat < 0 && v.lon < 0);  // true

If we want to find equivalents to these two functions in terms of reduce(), the two alternatives show nice symmetry:

arr.every(fn);// arr.reduce((x, y) => x && fn(y), true);arr.some(fn);