Can not declare or use a variable when it is defined in anther file

Question

I have two files in one folder.

# ls
a.ts  
b.ts
tsconfig.json

# tsc --version 
Version 3.0.1

And tsconfig.json is created by tsc -init without any change.

In a.ts, I define a variable.

let a = {
  name: 'hello'
}

In b.ts:

let a = {}

And I try to compiler:

# tsc
a.ts:1:5 - error TS2451: Cannot redeclare block-scoped variable 'a'.
// ... and some detail

So compiler throw this error, but it also gives me two compiled files.

# ls
a.js         
b.ts
a.ts         
tsconfig.json
b.js

This is detail in b.js:

"use strict";
var a = {};

Actual, compile is successful, but it still throws an error.

anther test

But if I try to use a in b.ts

console.log(a.name)

Compiler is fine, but failed when I try to execute the code.

So, I am confused. I can not declare or use a as a variable name in b.ts when I define this in anther file?

This is default tsconf.json

{
  "compilerOptions": {
    /* Basic Options */
    "target": "es5",                          /* Specify ECMAScript target version: 'ES3' (default), 'ES5', 'ES2015', 'ES2016', 'ES2017','ES2018' or 'ESNEXT'. */
    "module": "commonjs",                     /* Specify module code generation: 'none', 'commonjs', 'amd', 'system', 'umd', 'es2015', or 'ESNext'. */
    // "lib": [],                             /* Specify library files to be included in the compilation. */
    // "allowJs": true,                       /* Allow javascript files to be compiled. */
    // "checkJs": true,                       /* Report errors in .js files. */
    // "jsx": "preserve",                     /* Specify JSX code generation: 'preserve', 'react-native', or 'react'. */
    // "declaration": true,                   /* Generates corresponding '.d.ts' file. */
    // "declarationMap": true,                /* Generates a sourcemap for each corresponding '.d.ts' file. */
    // "sourceMap": true,                     /* Generates corresponding '.map' file. */
    // "outFile": "./",                       /* Concatenate and emit output to single file. */
    // "outDir": "./",                        /* Redirect output structure to the directory. */
    // "rootDir": "./",                       /* Specify the root directory of input files. Use to control the output directory structure with --outDir. */
    // "composite": true,                     /* Enable project compilation */
    // "removeComments": true,                /* Do not emit comments to output. */
    // "noEmit": true,                        /* Do not emit outputs. */
    // "importHelpers": true,                 /* Import emit helpers from 'tslib'. */
    // "downlevelIteration": true,            /* Provide full support for iterables in 'for-of', spread, and destructuring when targeting 'ES5' or 'ES3'. */
    // "isolatedModules": true,               /* Transpile each file as a separate module (similar to 'ts.transpileModule'). */

    /* Strict Type-Checking Options */
    "strict": true,                           /* Enable all strict type-checking options. */
    // "noImplicitAny": true,                 /* Raise error on expressions and declarations with an implied 'any' type. */
    // "strictNullChecks": true,              /* Enable strict null checks. */
    // "strictFunctionTypes": true,           /* Enable strict checking of function types. */
    // "strictPropertyInitialization": true,  /* Enable strict checking of property initialization in classes. */
    // "noImplicitThis": true,                /* Raise error on 'this' expressions with an implied 'any' type. */
    // "alwaysStrict": true,                  /* Parse in strict mode and emit "use strict" for each source file. */

    /* Additional Checks */
    // "noUnusedLocals": true,                /* Report errors on unused locals. */
    // "noUnusedParameters": true,            /* Report errors on unused parameters. */
    // "noImplicitReturns": true,             /* Report error when not all code paths in function return a value. */
    // "noFallthroughCasesInSwitch": true,    /* Report errors for fallthrough cases in switch statement. */

    /* Module Resolution Options */
    // "moduleResolution": "node",            /* Specify module resolution strategy: 'node' (Node.js) or 'classic' (TypeScript pre-1.6). */
    // "baseUrl": "./",                       /* Base directory to resolve non-absolute module names. */
    // "paths": {},                           /* A series of entries which re-map imports to lookup locations relative to the 'baseUrl'. */
    // "rootDirs": [],                        /* List of root folders whose combined content represents the structure of the project at runtime. */
    // "typeRoots": [],                       /* List of folders to include type definitions from. */
    // "types": [],                           /* Type declaration files to be included in compilation. */
    // "allowSyntheticDefaultImports": true,  /* Allow default imports from modules with no default export. This does not affect code emit, just typechecking. */
    "esModuleInterop": true                   /* Enables emit interoperability between CommonJS and ES Modules via creation of namespace objects for all imports. Implies 'allowSyntheticDefaultImports'. */
    // "preserveSymlinks": true,              /* Do not resolve the real path of symlinks. */

    /* Source Map Options */
    // "sourceRoot": "",                      /* Specify the location where debugger should locate TypeScript files instead of source locations. */
    // "mapRoot": "",                         /* Specify the location where debugger should locate map files instead of generated locations. */
    // "inlineSourceMap": true,               /* Emit a single file with source maps instead of having a separate file. */
    // "inlineSources": true,                 /* Emit the source alongside the sourcemaps within a single file; requires '--inlineSourceMap' or '--sourceMap' to be set. */

    /* Experimental Options */
    // "experimentalDecorators": true,        /* Enables experimental support for ES7 decorators. */
    // "emitDecoratorMetadata": true,         /* Enables experimental support for emitting type metadata for decorators. */
  }
}

Answer 1

...but failed when I try to execute the code.

That tells us that you're not running a.js before running b.js in the same environment. You can't just run b.js in isolation, a.js has to be run first so that a is declared and initialized.

If you do that, console.log(a.name) in b.js works just fine.

Using TypeScript v3 with your tsconfig.json, a.ts is compiled to this JavaScript:

"use strict";
var a = {
    name: 'hello'
};

and b.ts (the version with the console.log) is compiled to this JavaScript:

"use strict";
console.log(a.name);

Running a.js and then running b.js in the same environment successfully outputs the name. Live example

The reason you can't redeclare a in b.ts is that both a.ts and b.ts are evaluated at global scope, and you can't have let a at global scope in two places, because redeclaring variables with let is an error.

Answer 2

If your file contains no exports it is considered a script file, and all symbols declared inside it are in the global scope. This is probably why you are getting the error, you are defining the same variable in multiple files typescript considers script files.

You can either use namespaces (not recommended) or you could add a module system and use export al least once in your file.

If you are already using a module system, and just forgot to add an export you can add the isolatedModules to ensure all files are required to have at least one export