I don’t see any mention of performance here. Please see the concerns in the issue.

You're right, sorry.

In the usual code execution only one if-clause extra is checked:

if(Array.isArray(arguments_[0])) {
    ... // the new code
}

This clause is only triggered when the function (e.g. chalk.red) is called with an array argument. This (only) happens when used as a template literal. So the speed of existing code is not impacted.

if branches are not free. Please do a proper performance benchmark.

Using 'npm run bench'. These are the results

(nested_template_literals)
      24,755,209 op/s » 1 style
      21,323,845 op/s » 2 styles
      20,138,529 op/s » 3 styles
      23,569,342 op/s » cached: 1 style
      22,033,244 op/s » cached: 2 styles
      19,742,892 op/s » cached: 3 styles
      11,052,769 op/s » cached: 1 style with newline
       5,337,312 op/s » cached: 1 style nested intersecting
      14,257,177 op/s » cached: 1 style nested non-intersecting

(master)
      25,268,827 op/s » 1 style (2.1% faster)
      23,094,279 op/s » 2 styles (8.3% faster)
      20,585,710 op/s » 3 styles (2.2% faster)
      25,472,874 op/s » cached: 1 style (8.1% faster)
      22,120,499 op/s » cached: 2 styles (0.4% faster)
      20,772,105 op/s » cached: 3 styles (5.2% faster)
      11,559,382 op/s » cached: 1 style with newline (4.6% faster)
       5,340,114 op/s » cached: 1 style nested intersecting (0.05% faster)
      14,327,385 op/s » cached: 1 style nested non-intersecting (0.5% faster)

Are there still issues with this PR?

Can you add some tests to the benchmark using template literals and nesting? Could be useful for the future.

This also needs to be documented in the readme and index.d.ts.

• index.d.ts already allowed template literals for chained styles. This is now also reflected in index.test-d.ts.
• The readme contains an explicit example of the use of chalk.bold.rgb(10, 100, 200)`Hello!`
• Some benchmarks are added for template literals. Parsing templates is quite expensive so I've lowered the number of iterations for these two benchmarks
• Two complex chained template literals are tested.

index.d.ts already allowed template literals for chained styles.

I meant docs-wise. The readme and index.d.ts should be in sync docs-wise.

Is it okay like this?

@toonijn I don't understand your question. I already commented what needs to be done.

I think I've implemented the changes in my latest commit.
What docs should I change more?

I already commented that. What you added to the readme, you need to add to index.d.ts too (docs-wise).

@sindresorhus A small example is included in index.d.ts as well.

I'm ok with this.