Laravel 5.4 : Uncaught ReferenceError: Echo is not defined

I was trying to laravel echo with pusher in 5.4 for boradcasting a event. But the problem i'm facing I can't broadcast the event properly. Instead of it I'm getting the following Error>>

Uncaught ReferenceError: Echo is not defined

Here is the code in app.js:

created(){
            axios.get('messages').then(response => {
               // console.log(response);
                this.messages = response.data;
            });

            Echo.join('chatroom')
            .here((users)=> {
                this.usersInRoom = users;
            })
            .joining((user)=>{
                this.usersInRoom.push(user);
            })
           .leaving((user)=>{
                this.usersInRoom = this.usersInRoom.filter(u=>u !=user)
           })
            .listen('messagePosted',(e)=>{
                this.messages.push({
                    message:e.message.message,
                    user:e.user
                });
             //  console.log(e);
                });
    }

Can anyone suggest what's the Error with the Echo please?

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