Assignment inside function call?

[pintariching]

What is this line here doing?

const identifier = this.template.slice(this.index, this.index = i);

I can't figure out what the this.index = 1 means. Does it assign i to this.index?

[linrium]

for example, this.index equals 1, and I equal 2, then the slice method will be .slice(1, 2). This is a short syntax.

[pintariching]

Here's a fiddle. Appearently this does assign i to this.index and uses this new value as a parameter to the .slice function.