flatten.js

// Internal implementation of a recursive `flatten` function.

var property = require('./property');

// Helper for collection methods to determine whether a collection
// should be iterated as an array or as an object.
// Related: http://people.mozilla.org/~jorendorff/es6-draft.html#sec-tolength
// Avoids a very nasty iOS 8 JIT bug on ARM-64. #2094
var MAX_ARRAY_INDEX = Math.pow(2, 53) - 1;
var getLength = property('length');
var isArrayLike = function(collection) {
    var length = getLength(collection);
    return typeof length === 'number' && length >= 0 && length <= MAX_ARRAY_INDEX;
};

function isArguments(value) {
    return Object.prototype.toString.call(value) === '[object Arguments]';
}

var flatten = function(input, shallow, strict, output) {
    output = output || [];
    var idx = output.length;
    for (var i = 0, length = getLength(input); i < length; i++) {
        var value = input[i];
        if (isArrayLike(value) && (Array.isArray(value) || isArguments(value))) {
            //flatten current level of array or arguments object
            if (shallow) {
                var j = 0, len = value.length;
                while (j < len) output[idx++] = value[j++];
            } else {
                flatten(value, shallow, strict, output);
                idx = output.length;
            }
        } else if (!strict) {
            output[idx++] = value;
        }
    }
    return output;
};

module.exports = function (array, shallow) {
    return flatten(array, shallow, false);
};