cannot redeclare block scoped variable (typescript)

问题

I'm building a node app, and inside each file in .js used to doing this to require in various packages.

let co = require("co");

But getting

etc. So using typescript it seems there can only be one such declaration/require across the whole project? I'm confused about this as I thought let was scoped to the current file.

I just had a project that was working but after a refactor am now getting these errors all over the place.

Can someone explain?

回答1:

Regarding the error itself, let is used to declare local variables that exist in block scopes instead of function scopes. It's also more strict than var, so you can't do stuff like this:

if (condition) {
    let a = 1;
    ...
    let a = 2;
}

Also note that case clauses inside switch blocks don't create their own block scopes, so you can't redeclare the same local variable across multiple cases without using {} to create a block each.

As for the import, you are probably getting this error because TypeScript doesn't recognize your files as actual modules, and seemingly model-level definitions end up being global definitions for it.

Try importing an external module the standard ES6 way, which contains no explicit assignment, and should make TypeScript recognize your files correctly as modules:

import * as co from "./co"

This will still result in a compile error if you have something named co already, as expected. For example, this is going to be an error:

import * as co from "./co"; // Error: import definition conflicts with local definition
let co = 1;

If you are getting an error "cannot find module co"...

TypeScript is running full type-checking against modules, so if you don't have TS definitions for the module you are trying to import (e.g. because it's a JS module without definition files), you can declare your module in a .d.ts definition file that doesn't contain module-level exports:

declare module "co" {
    declare var co: any;
    export = co;
}

回答2:

The best explanation I could get is from Tamas Piro's post.

TLDR; TypeScript uses the DOM typings for the global execution environment. In your case there is a 'co' property on the global window object.

To solve this:

Rename the variable, or

Use TypeScript modules, and add an empty export{}:

export{};

or

Configure your compiler options by not adding DOM typings:

Edit tsconfig.json in the TypeScript project directory.

{
    "compilerOptions": {
        "lib": ["es6"]
      }
}

回答3:

I was receiving this similar error when compiling my Node.JS Typescript application:

node_modules/@types/node/index.d.ts:83:15 - error TS2451: Cannot redeclare block-scoped variable 'custom'.

The fix was to remove this:

"files": [
  "./node_modules/@types/node/index.d.ts"
]

and to replace it with this:

"compilerOptions": {
  "types": ["node"]
}

回答4:

Use IIFE(Immediately Invoked Function Expression), IIFE

(function () {
    all your code is here...

 })();

回答5:

I got the same problem, and my solution looks like this:

// *./module1/module1.ts*
export module Module1 {
    export class Module1{
        greating(){ return 'hey from Module1'}
    }
}


// *./module2/module2.ts*
import {Module1} from './../module1/module1';

export module Module2{
    export class Module2{
        greating(){
            let m1 = new Module1.Module1()
            return 'hey from Module2 + and from loaded Model1: '+ m1.greating();
        }
    }
}

Now we can use it on the server side:

// *./server.ts*
/// <reference path="./typings/node/node.d.ts"/>
import {Module2} from './module2/module2';

export module Server {
    export class Server{
        greating(){
            let m2 = new Module2.Module2();
            return "hello from server & loaded modules: " + m2.greating();
        }
    }
}

exports.Server = Server;

// ./app.js
var Server = require('./server').Server.Server;
var server = new Server();
console.log(server.greating());

And on the client side too:

// *./public/javscripts/index/index.ts*

import {Module2} from './../../../module2/module2';

document.body.onload = function(){
    let m2 = new Module2.Module2();
    alert(m2.greating());
}

// ./views/index.jade
extends layout

block content
  h1= title
  p Welcome to #{title}
  script(src='main.js')
  //
    the main.js-file created by gulp-task 'browserify' below in the gulpfile.js

And, of course, a gulp-file for all of this:

// *./gulpfile.js*
var gulp = require('gulp'),
    ts = require('gulp-typescript'),
    runSequence = require('run-sequence'),
    browserify = require('gulp-browserify'),
    rename = require('gulp-rename');

gulp.task('default', function(callback) {

    gulp.task('ts1', function() {
        return gulp.src(['./module1/module1.ts'])
            .pipe(ts())
            .pipe(gulp.dest('./module1'))
    });

    gulp.task('ts2', function() {
        return gulp.src(['./module2/module2.ts'])
            .pipe(ts())
            .pipe(gulp.dest('./module2'))
    });

    gulp.task('ts3', function() {
        return gulp.src(['./public/javascripts/index/index.ts'])
            .pipe(ts())
            .pipe(gulp.dest('./public/javascripts/index'))
    });

    gulp.task('browserify', function() {
        return gulp.src('./public/javascripts/index/index.js', { read: false })
            .pipe(browserify({
                insertGlobals: true
            }))
            .pipe(rename('main.js'))
            .pipe(gulp.dest('./public/javascripts/'))
    });

    runSequence('ts1', 'ts2', 'ts3', 'browserify', callback);
})

Updated. Of course, it's not neccessary to compile typescript files separatly. runSequence(['ts1', 'ts2', 'ts3'], 'browserify', callback) works perfect.

回答6:

I got this error when upgrading

gulp-typescript 3.0.2 → 3.1.0

Putting it back to 3.0.2 fixed it

来源：https://stackoverflow.com/questions/35758584/cannot-redeclare-block-scoped-variable-typescript

标签

typescript

require