I am having a bit of difficulty understanding a few lines of code from page 108 of Marijn Haverbeke's "Eloquent JavaScript" book.
Namely, in the example below, I don't understand what "element = (x, y) => undefined" in the constructor parameter list is doing, when the Matrix object is instantiated with "let matrix = new Matrix(3, 3, (x, y) => value ${x}, ${y});"
Can some step-by-step it for me? It seems like we are passing a function to the constructor, but I don't get why the constructor parameter list is setup the way it is with another arrow function.
var Matrix = class Matrix {
constructor(width, height, element = (x, y) => undefined) {
this.width = width;
this.height = height;
this.content = [];
for (let y = 0; y < height; y++) {
for (let x = 0; x < width; x++) {
this.content[y * width + x] = element(x, y);
}
}
}
get(x, y) {
return this.content[y * this.width + x];
}
set(x, y, value) {
this.content[y * this.width + x] = value;
}
}
var MatrixIterator = class MatrixIterator {
constructor(matrix) {
this.x = 0;
this.y = 0;
this.matrix = matrix;
}
next() {
if (this.y == this.matrix.height) return {done: true};
let value = {x: this.x,
y: this.y,
value: this.matrix.get(this.x, this.y)};
this.x++;
if (this.x == this.matrix.width) {
this.x = 0;
this.y++;
}
return {value, done: false};
}
}
Matrix.prototype[Symbol.iterator] = function() {
return new MatrixIterator(this);
};
let matrix = new Matrix(3, 3, (x, y) => `value ${x}, ${y}`);
for (let {x, y, value} of matrix) {
console.log(x, y, value);
}
