Clarify + usage

[sdavids]

📚 Docs or angular.io bug report

Description

The documentation is aimed at newcomers to Angular/JavaScript--so please clarify why + is used.

🔬 Minimal Reproduction

What's the affected URL?

https://angular.io/start/start-routing#using-route-information

ngOnInit() {
  this.route.paramMap.subscribe(params => {
    this.product = products[+params.get('productId')];
  });
}

products[+params.get('productId')]

vs.

products[params.get('productId')]

[AnthonyLenglet]

Duplicate of #33001

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