I'm trying to implement reduce() from scratch. I was able to get it working, but how does javascript know what "array" is even though I never defined it anywhere?
function reduce(callback, initialVal) {
var accumulator = (initialVal === undefined) ? undefined : initialVal;
for (var i=0; i<array.length; i++) {
if (accumulator !== undefined) {
accumulator = callback(accumulator, array[i], i, array);
} else {
accumulator = array[i]
}
}
return accumulator;
};
// testing a basic sum
arr = [1,2,3]
arr.reduce( (accumulator, elem) => accumulator+=elem )
EDIT: I got it working :D I Changed 'array' to "this" since I was creating a new method under Array.prototype.
Array.prototype.myReduce = function(callback, initialVal) {
var accumulator = (initialVal !== undefined) ? initialVal : undefined;
for (var i=0; i<this.length; i++) {
if (accumulator !== undefined) {
accumulator = callback(accumulator, this[i], i, this);
} else {
accumulator = this[i]
}
}
return accumulator;
};
arr.myReduce( (accumulator, elem) => accumulator+=elem )
arr.myReduce( (accumulator, elem) => accumulator+=elem , 100)
I'm trying to implement reduce() from scratch. I was able to get it working, but how does javascript know what "array" is even though I never defined it anywhere?
function reduce(callback, initialVal) {
var accumulator = (initialVal === undefined) ? undefined : initialVal;
for (var i=0; i<array.length; i++) {
if (accumulator !== undefined) {
accumulator = callback(accumulator, array[i], i, array);
} else {
accumulator = array[i]
}
}
return accumulator;
};
// testing a basic sum
arr = [1,2,3]
arr.reduce( (accumulator, elem) => accumulator+=elem )
EDIT: I got it working :D I Changed 'array' to "this" since I was creating a new method under Array.prototype.
Array.prototype.myReduce = function(callback, initialVal) {
var accumulator = (initialVal !== undefined) ? initialVal : undefined;
for (var i=0; i<this.length; i++) {
if (accumulator !== undefined) {
accumulator = callback(accumulator, this[i], i, this);
} else {
accumulator = this[i]
}
}
return accumulator;
};
arr.myReduce( (accumulator, elem) => accumulator+=elem )
arr.myReduce( (accumulator, elem) => accumulator+=elem , 100)
-
you need to create array.prototype.And also
array.reduceoverwrite the default js is bad practise.its affected the library of you are included.Better use different name instead js default – prasanth Commented Apr 16, 2019 at 2:51 -
Are you sure YOUR code is getting called and not built in
reducethat is onArray'sprototype? – JohanP Commented Apr 16, 2019 at 2:51 -
[and]tells JS that it's an array – Anand G Commented Apr 16, 2019 at 2:51 - Possible duplicate of adding custom functions into Array.prototype – prasanth Commented Apr 16, 2019 at 2:55
- 1 @prasanth you guys are right, it's most likely calling the native reduce() and not the one I just created...if I change the function to "myReduce()" it throws the error "TypeError: arr.myReduce is not a function". I'll retry by creating it under Array.prototype! – tbd_ Commented Apr 16, 2019 at 3:04
2 Answers
Reset to default 16You are pretty close. The one insight that seems to be missing is that inside your function, this will be defined as the array your function was called on. So you can use this in the places you're currently using array, which in your function will be undefined (as you suspected). You probably also want to start the loop in a different place depending on whether the initial value was passed in. For example:
function reduce(callback, initialVal) {
var accumulator = ( initialVal === undefined) ? this[0] : initialVal;
var start = (initialVal === undefined) ? 1 : 0
for (var i = start; i < this.length; i++) {
accumulator = callback(accumulator, this[i])
}
return accumulator;
};
Array.prototype.myReduce = reduce
// no init value
console.log([1, 2, 3].myReduce((sum, curr) => sum + curr))
// init value:
console.log([1, 2, 3].myReduce((sum, curr) => sum + curr, 1000))Crips and clear
Array.prototype.myReduce = function (callback, iterator) {
// Initialize the iterator based on the type of the first element
if (typeof this[0] === "number" && iterator === undefined) {
iterator = 0;
} else if (typeof this[0] === "string" && iterator === undefined) {
iterator = "";
}
// Iterate through the array and apply the callback
for (let i = 0; i < this.length; i++) {
iterator = callback(iterator, this[i]);
}
return iterator; // Return the final accumulated value
};