Documentation for Typescript usage needs to highlight that the spec should be declared "as const".

[unphased]

See the following comment. Until I saw this explanation I was going crazy looking for how to import types like AnyMark, which typescript was trying to tell me my "bar" mark type was invalid for.

import {compile} from 'vega-lite';

const vegaLiteSpec = {
  $schema: 'https://vega.github.io/schema/vega-lite/v4.json',
  data: {
    values: [
      {a: 'A', b: 28},
      {a: 'B', b: 55},
      {a: 'C', b: 43},
      {a: 'D', b: 91},
      {a: 'E', b: 81},
      {a: 'F', b: 53},
      {a: 'G', b: 19},
      {a: 'H', b: 87},
      {a: 'I', b: 52}
    ]
  },
  mark: 'bar',
  encoding: {
    x: {field: 'a', type: 'nominal', axis: {labelAngle: 0}},
    y: {field: 'b', type: 'quantitative'}
  }
} as const;

const vegaSpec = compile(vegaLiteSpec).spec;

The issue without as const is that Typescript widens the type of "bar" to string rather than the literal "bar".

Originally posted by @domoritz in #7127 (comment)