Generator functions
JavaScript ES6 generators allow you to define functions that can be exited and later re-entered, while retaining their context (variable bindings). They are defined using function*
(function
keyword followed by an asterisk) and use yield
expressions to return their result. For example:
代码实现
function* generateRange(end, start = 0, step = 1) {
let x = start - step;
while(x < end - step) yield x += step;
}
const gen5 = generateRange(5);
let x = gen5.next();
while (!x.done) {
console.log(x.value);
x = gen5.next();
} // Logs: 0, 1, 2, 3, 4
In the above example, we define a generator function, generateRange
, which will return each value between start
and end
, incrementing by step
each time. We use the generator object to call Generator.prototype.next()
until it returns {value: undefined, done: true}
to iterate over the values the generator produces.
Symbol.iterator
Symbol.iterator
specifies the default iterator for an object. Oftentimes, Symbol.iterator
is implemented using a generator function. For example:
使用样例
const iterableXx = {
[Symbol.iterator]: function* () {
yield 1;
yield 2;
}
};
console.log([...iterableX]); // [1, 2]
As you can see in this example, the object is made iterable by assigning a generator function to its Symbol.iterator
property. This can come especially handy, if you want to iterate over some arbitrary data or create an object that is iterable and uses a generator function under the hood.
Putting it all together
Knowing how both concepts work, we can combine them to create a range generator, similar to Python or Ruby’s ranges:
const range = (end, start = 0, step = 1) => {
function* generateRange() {
let x = start - step;
while(x < end - step) yield x += step;
}
return {
[Symbol.iterator]: generateRange
};
}
console.log([...range(7)]); // [0, 1, 2, 3, 4, 5, 6]
for (let i of range(8, 2, 2)) console.log(i); // Logs: 2, 4, 6
翻译自:https://www.30secondsofcode.org/js/s/generator-for-range