TS Error: Type 'string' is not an array type or a string type. How a string is not a string?

Question

TS throws strange error:

Error:(125, 18) TS2569: Type 'string' is not an array type or a string type. Use compiler option '--downlevelIteration' to allow iterating of iterators.

How comes a string is not a string?

I want to see how TS is going to compile spread operator for a string.

My code in browser console. A string is broken up into characters:

> s = 'abcdef';
> r = [...s];
< (6) ["a", "b", "c", "d", "e", "f"]

My code in TS:

const s: string = 'abcdef';
const res = [...s]; // <= Error: Type 'string' is not an array type or a string type
console.log(res);

Why?

TS version:

  "dependencies": {
    "typescript": "^3.5.3"
  }

UPD:

@VtoCorleone A screenshot

UPD:

My tsconfig.json

{
  "compilerOptions": {
    "target": "es5",
    "lib": [
      "dom",
      "dom.iterable",
      "esnext"
    ],
    "downlevelIteration": false,
    "allowJs": true,
    "skipLibCheck": false,
    "esModuleInterop": true,
    "allowSyntheticDefaultImports": true,
    "alwaysStrict": true,
    "strict": true,
    "forceConsistentCasingInFileNames": true,
    "module": "esnext",
    "moduleResolution": "node",
    "resolveJsonModule": true,
    "isolatedModules": false,
    "noEmit": false,
    "sourceMap": true,
    "baseUrl": "./",
    "jsx": "preserve"
  },
  "compileOnSave": true,
  "files": [
    "sample.ts"
  ],
  "exclude": [
    "node_modules",
    "**/*.spec.ts"
  ]
}

Might be a stupid question, but have you tried adding "downlevelIteration": true to your tsconfig? — OliverRadini
– OliverRadini, Commented Jul 11, 2019 at 13:45
@OliverRadini, Yes, I did both, true and false. TS doesn't blow when true as expected. But why is it complaining a string is not a string? — Green
– Green, Commented Jul 11, 2019 at 13:51
I just copy and pasted your exact example with TS 3.5.1 and got no warnings or errors. — VtoCorleone
– VtoCorleone, Commented Jul 11, 2019 at 14:35
What are you targeting? In the TS playground I get errors if I target ES5 or below... — Heretic Monkey
– Heretic Monkey, Commented Jul 11, 2019 at 16:11
@HereticMonkey Yes, indeed "target": "es5",. I've updated the question with my tsconfig.json — Green
– Green, Commented Jul 12, 2019 at 8:44

Andy Mardell · Accepted Answer · 2020-01-09 11:06:00Z

31

To expand on Heretic Monkey's comment:

Changing target from es5 to es2015 or es6 fixes the issue. Here's my full tsconfig.json for clarity:

{
  "compilerOptions": {
    "target": "es2015",
    "lib": [
      "dom",
      "dom.iterable",
      "esnext"
    ],
    "allowJs": true,
    "skipLibCheck": true,
    "strict": true,
    "forceConsistentCasingInFileNames": true,
    "noEmit": true,
    "esModuleInterop": true,
    "module": "esnext",
    "moduleResolution": "node",
    "resolveJsonModule": true,
    "isolatedModules": true,
    "jsx": "preserve",
  },
  "exclude": [
    "node_modules"
  ],
  "include": [
    "next-env.d.ts",
    "**/*.ts",
    "**/*.tsx"
  ]
}

Working example

Side note: "downlevelIteration": true also fixed it, but that doesn't seem like the correct solution to me.

answered Jan 9, 2020 at 11:06

Andy Mardell

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Comments

Kamil Kiełczewski · Accepted Answer · 2020-04-29 19:23:33Z

4

I have Angular-CLI (8) - in my case modification tsconfig.json in key "target": "es2015" (or es2018) not helps at all. Only adding "downlevelIteration": true helps. Here is my whole file

{
  "compileOnSave": false,
  "compilerOptions": {
    "baseUrl": "./",
    "outDir": "./dist/out-tsc",
    "sourceMap": true,
    "declaration": false,
    "module": "esnext",
    "moduleResolution": "node",
    "emitDecoratorMetadata": true,
    "experimentalDecorators": true,
    "importHelpers": true,
    "downlevelIteration": true,
    "target": "es2015",
    "typeRoots": [
      "node_modules/@types"
    ],
    "lib": [
      "es2018",
      "dom"
    ]
  }
}

answered Apr 29, 2020 at 19:23

Kamil Kiełczewski

93.8k34 gold badges401 silver badges375 bronze badges

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