If you're using vue-cli 3.x, you can simply pass the port to the npm command like so:

npm run serve -- --port 3000

Then visit http://localhost:3000/

Late to the party, but I think it's helpful to consolidate all these answers into one outlining all options.

Separated in Vue CLI v2 (webpack template) and Vue CLI v3, ordered by precedence (high to low).

Vue CLI v3

• package.json: Add port option to serve script: scripts.serve=vue-cli-service serve --port 4000
• CLI Option --port to npm run serve, e.g. npm run serve -- --port 3000. Note the --, this makes passes the port option to the npm script instead of to npm itself. Since at least v3.4.1, it should be e.g. vue-cli-service serve --port 3000.
• Environment Variable $PORT, e.g. PORT=3000 npm run serve
• .env Files, more specific envs override less specific ones, e.g. PORT=3242
• vue.config.js, devServer.port, e.g. devServer: { port: 9999 }

References:

• https://cli.vuejs.org/config/#devserver
• https://cli.vuejs.org/config/#vue-config-js
• https://cli.vuejs.org/guide/mode-and-env.html

Vue CLI v2 (deprecated)

• Environment Variable $PORT, e.g. PORT=3000 npm run dev
• /config/index.js: dev.port

References:

• http://vuejs-templates.github.io/webpack/
• https://github.com/vuejs-templates/webpack/blob/develop/template/build/webpack.dev.conf.js#L35

As the time of this answer's writing (May 5th 2018), vue-cli has its configuration hosted at <your_project_root>/vue.config.js. To change the port, see below:

// vue.config.js
module.exports = {
  // ...
  devServer: {
    open: process.platform === 'darwin',
    host: '0.0.0.0',
    port: 8080, // CHANGE YOUR PORT HERE!
    https: false,
    hotOnly: false,
  },
  // ...
}

Full vue.config.js reference can be found here: https://cli.vuejs.org/config/#global-cli-config

Note that as stated in the docs, “All options for webpack-dev-server” (https://webpack.js.org/configuration/dev-server/) is available within the devServer section.

The port for the Vue-cli webpack template is found in your app root's myApp/config/index.js.

All you have to do is modify the port value inside the dev block:

 dev: {
    proxyTable: {},
    env: require('./dev.env'),
    port: 4545,
    assetsSubDirectory: 'static',
    assetsPublicPath: '/',
    cssSourceMap: false
  }

Now you can access your app with localhost:4545

also if you have .env file better to set it from there

Another option if you're using vue cli 3 is to use a config file. Make a vue.config.js at the same level as your package.json and put a config like so:

module.exports = {
  devServer: {
    port: 3000
  }
}

Configuring it with the script:

npm run serve --port 3000

works great but if you have more config options I like doing it in a config file. You can find more info in the docs.

Best way is to update the serve script command in your package.json file. Just append --port 3000 like so:

"scripts": {
  "serve": "vue-cli-service serve --port 3000",
  "build": "vue-cli-service build",
  "inspect": "vue-cli-service inspect",
  "lint": "vue-cli-service lint"
},

First Option:

OPEN package.json and add "--port port-no" in "serve" section.

Just like below, I have done it.

{
  "name": "app-name",
  "version": "0.1.0",
  "private": true,
  "scripts": {
    "serve": "vue-cli-service serve --port 8090",
    "build": "vue-cli-service build",
    "lint": "vue-cli-service lint"
}

Second Option: If You want through command prompt

npm run serve --port 8090

If you want to change the localhost port, you can change scripts tag in package.json:

 "scripts": {
    "serve": "vue-cli-service serve --port 3000",
    "build": "vue-cli-service build",
    "lint": "vue-cli-service lint"
  },

In the webpack.config.js:

module.exports = {
  ......
  devServer: {
    historyApiFallback: true,
    port: 8081,   // you can change the port there
    noInfo: true,
    overlay: true
  },
  ......
}

You can change the port in the module.exports -> devServer -> port.

Then you restrat the npm run dev. You can get that.

Oh my God! It is not that much complicated, with these answers which also works. However, other answers tho this question also works well.

If you really want to use the vue-cli-service and if you want to have the port setting in your package.json file, which your 'vue create <app-name>' command basically creates, you can use the following configuration: --port 3000. So the whole configuration of your script would be like this:

...
"scripts": {
"serve": "vue-cli-service serve --port 3000",
"build": "vue-cli-service build",
"lint": "vue-cli-service lint"
},
...

I am using @vue/cli 4.3.1 (vue --version) on a macOS device.

I have also added the vue-cli-service reference: https://cli.vuejs.org/guide/cli-service.html

An alternative approach with vue-cli version 3 is to add a .env file in the root project directory (along side package.json) with the contents:

PORT=3000

Running npm run serve will now indicate the app is running on port 3000.

There are a lot of answers here varying by version, so I thought I'd confirm and expound upon Julien Le Coupanec's answer above from October 2018 when using the Vue CLI. In the most recent version of Vue.js as of this post - vue@2.6.10 - the outlined steps below made the most sense to me after looking through some of the myriad answers in this post. The Vue.js documentation references pieces of this puzzle, but isn't quite as explicit.

1. Open the package.json file in the root directory of the Vue.js project.
2. Search for "port" in the package.json file.

3. Upon finding the following reference to "port", edit the serve script element to reflect the desired port, using the same syntax as shown below:

   "scripts": {
     "serve": "vue-cli-service serve --port 8000",
     "build": "vue-cli-service build",
     "lint": "vue-cli-service lint"
   }
   

4. Make sure to re-start the npm server to avoid unnecessary insanity.

The documentation shows that one can effectively get the same result by adding --port 8080 to the end of the npm run serve command like so: npm run serve --port 8080. I preferred editing the package.json directly to avoid extra typing, but editing npm run serve --port 1234 inline may come in handy for some.

To change the port (NPM), go to package.json. In scripts write your own script, for example:

"start": "npm run serve --port [PORT YOU WANT]"

After that you can start with npm start

If you use yarn:

yarn serve --port 3000

Add the PORT envvariable to your serve script in package.json:

"serve": "PORT=4767 vue-cli-service serve",

1. open package.json
2. add script named serve, "serve": "Vue-cli-service serve --port 8081"
3. npm run serve you will server run 8081

{
  "name": "app-name",
  "version": "0.1.0",
  "private": true,
  "scripts": {
    "serve": "vue-cli-service serve --port 8081", 
    "build": "vue-cli-service build",
    "lint": "vue-cli-service lint"
  }
}

In my vue project in visual studio code, I had to set this in /config/index.js. Change it in the:

module.exports = {
    dev: {
          // Paths
          assetsSubDirectory: 'static',
          assetsPublicPath: '/',
          proxyTable: {},

          host: 'localhost', // can be overwritten by process.env.HOST
          port: 8090, // can be overwritten by process.env.PORT, if port is in use, a free one will be determined
          autoOpenBrowser: false,
          errorOverlay: true,
          notifyOnErrors: true,
          poll: false    
         }
}

If you want to change the port number temporarily, you can add a –port option to npm run serve command.

npm run serve -- --port 6058

You should be good with this:

"serve": "vue-cli-service serve --port 8081",

If you are running this via Visual Studio Community or Professional (maybe with a .Net Core project) you will find that no matter what steps you do, when you launch the solution that it uses 8080.

Well there is launch.json file you need to edit hidden in the .vscode directory. MS don't tell you about this at all and a file search does not seem to find it.

Go to node_modules/@vue/cli-service/lib/options.js
At the bottom inside the "devServer" unblock the codes
Now give your desired port number in the "port" :)

devServer: {
   open: process.platform === 'darwin',
   host: '0.0.0.0',
   port: 3000,  // default port 8080
   https: false,
   hotOnly: false,
   proxy: null, // string | Object
   before: app => {}
}