{1:’a’}[1] This syntax works in code but not in the browser’s developer tools?

I’ve taken on a new project,and there’s a line of code that looks like this:

typeName() {
      return {
        1: 'leads',
        2: 'cunstomer',
        3: 'business'
      }[this.optionValue]
    },

it works well.
I’ve never seen before.
Then I tried in developer tools,but it don’t work!
Can anyone tell me why?
enter image description here

{1:'a'}[1]

Developer tools will report the error:Uncaught SyntaxError: Unexpected token ‘:’