javascript range() generator modeled after python

range.js

//
//  range -- generator similar to python's range()
//
//  range() will generate 0..MAX_SAFE_INTEGER
//  range(end) will generate 0..(end-1)
//  range(start, end) will generate start..(end-1)
//  range(start, end, step) will step by step instead of 1
//  start can be larger than end if step is negative:
//      for example range(10, 2, -1)
//
function* range(...args) {
    let start = 0;
    let end = Number.MAX_SAFE_INTEGER;
    let step = 1;
    if (args.length === 1) {
        [end] = args;
    } else if (args.length > 1) {
        [start, end, step = 1] = args;
    }
    if (step > 0) {
        // stepping up
        while (start < end) {
            yield start;
            start += step;
        }
    } else {
        // stepping down
        while (start > end) {
            yield start;
            start += step;
        }
    }
}

// tests
for (x of range(5)) {
    console.log(`range(5): ${x}`);
}
for (x of range(6, 10)) {
    console.log(`range(6, 10): ${x}`);
}
for (x of range(10, 20, 3)) {
    console.log(`range(10, 20, 3): ${x}`);
}
for (x of range(10, 2, -1)) {
    console.log(`range(10, 2, -1): ${x}`);
}
for (x of range()) {
    console.log(`range() until x > 5: ${x}`);
    if (x > 5) {
        break;
    }
}
// why we use for...of instead of for...in.
// this generates the entire range into an array
// rather than letting it yield a value each iteration
for (x in [...range(5)]) {
    console.log(`in range(5): ${x}`);
}