Proposal: polyfill or covert [includes] method with target < es5

[e-cloud]

TypeScript Version: 2.3.2

Code

// A *self-contained* demonstration of the problem follows...
const str = "hello world"
if(str.includes("hello")) {
  // do something
}

with compile target as es5
Expected behavior:

var str = "hello world"
if(str.indexOf("hello") > -1) {
  // do something
}

Actual behavior:

var str = "hello world"
if(str.includes("hello")) {
  // do something
}

An unreasonable thing is some of members refuse to support that feature for ES2016 not being widely used in #2340. However, async and await features in ES2017 are supported by TS 2.3.

Therefore, i think it's time to revisit this requirement.
Otherwise, I have to use babel to transpile the ts output. That's really really ugly and stupid.

[RichiCoder1]

I'm curious, is there a reason you couldn't include a polyfill? As far as I'm aware, TypeScript only attempts to transpile language features, not library functions.

[mhegazy]

This has been one of the design goals to first impose no additional runtime cost. as a developer you have full control on what polyfills to include, and what to avoid. and second TS does make any type-directed emit. this allows you to use the compiler in a transpile mode where every file is processed in isolation.