Typescript: [BUG] VS2019 -> target = ES5 -> TS2354: tslib not found

[TobiasSekan]

TypeScript Version: 3.7.1-rc

Search Terms:
VS2019, for-in, es5, downlevelIteration, TS2354, tslib

Environment

• Microsoft Visual Studio Community 2019 Preview (2) Version 16.4.0 Preview 2.0
• TypeScript.MSBuild.3.7.1-rc (NuGet package)
• only AMD with require.js
• no babel, no webpack, no node, etc. (node are only used from VS2019)

Code

/**
 * Count the same characters from the left site of a given text
 * @param text The text that should contain the characters
 * @param characterToCount The character to count
 */
export function CountFromLeft(text: string, characterToCount: string): number
{
    let count = 0;

    for(let character of text)
    {
        if(character === characterToCount)
        {
            count++;
            continue;
        }
        else
        {
            break;
        }
    }

    return count;
}

tsconfig.json

{
    "compilerOptions":
    {
        "allowJs": false,
        "allowSyntheticDefaultImports": false,
        "allowUmdGlobalAccess": false,
        "allowUnreachableCode": false,
        "allowUnusedLabels": false,
        "alwaysStrict": true,
        // "baseUrl": "",
        // "charset": "",
        "checkJs": false,
        "composite": false,
        "declaration": false,
        // "declarationDir": "",
        "declarationMap": false,
        "disableSizeLimit": false,
        "diagnostics": false,
        "downlevelIteration": true,
        "emitBOM": true,
        "emitDeclarationOnly": false,
        "emitDecoratorMetadata": true,
        "esModuleInterop": false,
        "experimentalDecorators": true,
        "forceConsistentCasingInFileNames": true,
        "importHelpers": true,
        "incremental": false,
        "inlineSourceMap": true,
        "inlineSources": true,
        "isolatedModules": false,
        "jsx": "preserve",
        // "jsxFactory": "React.createElement",
        "keyofStringsOnly": false,
        // "lib": [],
        "listEmittedFiles": false,
        "listFiles": false,
        // "mapRoot": "",
        // "maxNodeModuleJsDepth": 0,
        "module": "amd",
        "moduleResolution": "node",
        "newLine": "CRLF",
        "noEmit": false,
        "noEmitHelpers": false,
        "noEmitOnError": true,
        "noErrorTruncation": true,
        "noFallthroughCasesInSwitch": true,
        "noImplicitAny": true,
        "noImplicitReturns": true,
        "noImplicitThis": true,
        "noImplicitUseStrict": false,
        "noLib": false,
        "noResolve": false,
        "noStrictGenericChecks": false,
        "noUnusedLocals": true,
        "noUnusedParameters": true,
        // "outDir": "",
        // "outFile": "",
        // "paths": {},
        // "plugins": [],
        "preserveConstEnums": false,
        "preserveSymlinks": true,
        "preserveWatchOutput": false,
        "pretty": true,
        // "reactNamespace": "",
        "removeComments": true,
        "resolveJsonModule": true,
        // "rootDir": "",
        // "rootDirs": [],
        // "skipDefaultLibCheck": false,
        "skipLibCheck": false,
        "sourceMap": false,
        "strict": true,
        "strictBindCallApply": true,
        "strictFunctionTypes": true,
        "strictNullChecks": true,
        "strictPropertyInitialization": true,
        "stripInternal": true,
        // "sourceRoot": "",
        "suppressExcessPropertyErrors": false,
        "suppressImplicitAnyIndexErrors": false,
        "target": "es5",
        "traceResolution": false,
        // "tsBuildInfoFile": "",
        // "typeRoots": [],
        // "types": [],
        "watch": false
    }
}

Expected behavior:
No error -> Build a ES5 compatible JavaScript function/file (working in IE 11)

Actual behavior:
Error: TS2354 (TS) This syntax requires an imported utility, but the "tslib" module was not found.

Related Issues:
possible: #17642

[AviVahl]

tsconfig contains "importHelpers": true, so you need to install tslib as a dependency of your project (e.g. npm i tslib or yarn add tslib) .
see: https://www.typescriptlang.org/docs/handbook/compiler-options.html

You can also set importHelpers to false (its default value), which would mean ES5 helpers will be inlined instead of being imported.

[TobiasSekan]

Thanks 👍

You can also set importHelpers to false (its default value), which would mean ES5 helpers will be inlined instead of being imported.

-> closed as not a bug -> missing documentation

[ittce]

in typescript version 3.9.3。 when I set importHelpers false。

[ittce]

The build result also import { __rest } from "tslib";
The tsconf.json like this:

{
    "compilerOptions": {
        "target": "es5",
        "module": "es2015",
        "moduleResolution": "node",
        "allowJs": false,
        "noEmitOnError": true,
        "downlevelIteration": true,
        "alwaysStrict": true,
        "outDir": "dist",
        "sourceMap": false,
        "declaration": false,
        "removeComments": false,
        "esModuleInterop":true,
        "strict": true,
        "noImplicitReturns": true,
        "noUnusedLocals": false,
        "noUnusedParameters": false,
        "noImplicitAny": true,
        "noImplicitThis": true,
        "jsx": "react-native",
        "resolveJsonModule": true,
        "experimentalDecorators": true,
        "lib": ["es6"],
        "importHelpers": false,
        "typeRoots": [
            "./node_modules/@qnpm",
            "./node_modules/@types"
        ],
        "types": [
            "node",
            "react",
            "react-native",
            "qunar-react-native-types"
        ]
    },
    "include": ["packages/**/*.ts", "packages/**/*.tsx"]
}

[ittce]

sorry。 I debug， in custom compile code。 The dever set Config default to true。