Typescript error: Property 'flat' does not exist on type '[string, unknown][]'

Question

I need to use.flat() on an array, but Typescript claims:

Property 'flat' does not exist on type '[string, unknown][]'

I've added "es2019 to my tsconfig file:

 "lib": ["es2015", "es2019", "dom"],

The function where flat() is used is:

 const legendSeries = (series) => { const formattedSeries = {}; series.map((serie) => Object.entries(serie).map((s) => { if (s[0] in formattedSeries) { return (formattedSeries[s[0]].count += s[1]); } return (formattedSeries[s[0]] = { id: s[0], label: s[0], count: s[1] }); }) ); return Object.entries(formattedSeries).flat().filter((_, i) => i % 2;== 0); };

The error persists. How to fix this (it's a React project in case it might be helpful)?

Answer 1

I think this issue is because typescript doesn't have access to array.flat(), in such a case it will throw an error of not have knowledge of flat(), as mentioned above by @codejockie. You need to update your config file like { "compilerOptions": { "target": "es5", "lib": [ "es2019" ] } }

we need to add es2019 or es2019.array to --lib setting for TypeScript so that it starts recognizing array.flat() and flatMap().

After making the changes. Please restart your project if the changes don't reflect.

Answer 2

Update your lib to this "lib": ["es2019", "es2017", "es7", "es6", "dom"] .

This is my config:

{
  "compilerOptions": {
    /* Basic Options */
    "target": "es5" /* Specify ECMAScript target version: 'ES3' (default), 'ES5', 'ES2015', 'ES2016', 'ES2017', 'ES2018', 'ES2019', 'ES2020', or 'ESNEXT'. */,
    "module": "commonjs" /* Specify module code generation: 'none', 'commonjs', 'amd', 'system', 'umd', 'es2015', 'es2020', or 'ESNext'. */,
    "resolveJsonModule": true,
    "lib": [
      "es2019",
      "es2017",
      "es7",
      "es6",
      "dom"
    ] /* Specify library files to be included in the compilation. */,
    "declaration": true /* Generates corresponding '.d.ts' file. */,
    "outDir": "dist" /* Redirect output structure to the directory. */,

    /* Strict Type-Checking Options */
    "strict": true /* Enable all strict type-checking options. */,
    "esModuleInterop": true /* Enables emit interoperability between CommonJS and ES Modules via creation of namespace objects for all imports. Implies 'allowSyntheticDefaultImports'. */,

    /* Advanced Options */
    "forceConsistentCasingInFileNames": true /* Disallow inconsistently-cased references to the same file. */,
    "baseUrl": ".",
    "paths": {
      "@/*": ["src/*"]
    }
  },
  "include": ["src/**/*.ts"],
  "exclude": ["node_modules", "dist", "examples", "**/*.spec.ts"]
}