Don't declare variable twice

// bad: declaring variable twice
function test() {
    "use strict";
    var a = 1,
        a = 2;
}

/*this will probably make your program not work as expected,
this can be fixed by renaming one of them
*/
function test() {
    "use strict";
    var a = 1,
        b = 2;
}

Read more about it: already defined variable(jshint)

References

Use const for all of your references; avoid using var

Why? This ensures that you can't reassign your references (mutation), which can lead to bugs and difficult to comprehend code.

// bad
var a = 1,
    b = 2;

// good
const a = 1,
      b = 2;

If you must mutate references, use let instead of var

Why? let is block-scoped rather than function-scoped like var.

  // bad
  var count = 1;
  if (true) {
    count += 1;
  }

  // good, use the let.
  let count = 1;
  if (true) {
    count += 1;
  }

Note that both let and const are block-scoped

// const and let only exist in the blocks they are defined in.
{
  let a = 1;
  const b = 1;
}
console.log(a); // ReferenceError
console.log(b); // ReferenceError

Read more about references: js best practices

Use the literal syntax for array creation

// bad
const items = new Array();

// good
const items = [];

Read more about it: js array best practices

Use the literal syntax for object creation.

// bad
const item = new Object();

// good
const item = {};

Read more about it: js object best practices