Setting up a variable with no operation assigned to it

Question

Setting up a variable with no operation assigned to it

As I delved into the Angular utils source code, I stumbled upon this interesting line:

export const NOOP: any = () => {};

It seems pretty straightforward - a variable that doesn't perform any operation. But then, within the same library, there is a method that makes use of this variable:

export function resolveViewDefinition(factory: ViewDefinitionFactory): ViewDefinition {
  let value: ViewDefinition = VIEW_DEFINITION_CACHE.get(factory) !;
  if (!value) {
    value = factory(() => NOOP);
    value.factory = factory;
    VIEW_DEFINITION_CACHE.set(factory, value);
  }
  return value;
}

What would happen if we were to comment out or remove this particular line within the block:

  if (!value) {
    // value = factory(() => NOOP);
    value.factory = factory;
    VIEW_DEFINITION_CACHE.set(factory, value);
  }

Can anyone provide more insight on this line:

value = factory(() => NOOP);

and explain its significance? While I can follow what it's doing, I'm curious about the potential consequences of excluding it.

angular typescript typescript2.0

Answer 1

Answer №1

resolveViewDefinition function is primarily used to retrieve view definitions. It first attempts to fetch the value from the cache, and if it is not found in the cache, then it proceeds to call the ViewDefinitionFactory.

The ViewDefinitionFactory requires a function as a parameter. But why?

/**
 * Factory for ViewDefinitions.
 * We use a function so we can reexecute it in case an error happens and use the given logger
 * function to log the error from the definition of the node, which is shown in all browser
 * logs.
 */
export interface ViewDefinitionFactory { (logger: NodeLogger): ViewDefinition; }

If we are simply obtaining the ViewDefinition, there's no need to log errors, so it is invoked with the NOOP function.

However, in cases where an error occurs during the execution of actions within the template while Angular is running, this factory utilizes the NodeLogger to pinpoint the node responsible for the error.

Check out this example on Plunker

@Component({
  selector: 'my-app',
  template: `<h2 (click)="x()">Hello</h2>`
})
export class App {}

Here is a snippet of the factory code:

function View_App_0(l) {
  return jit_viewDef1(0,[
      (l()(),jit_elementDef2(0,null,null,1,'h2',[],null,[[
        null,
        'click'
      ]
    ],function(v,en,$event) {
      var ad = true;
      var co = v.component;
      if (('click' === en)) {
        var pd_0 = (co.x() !== false);
        ad = (pd_0 && ad);
      }
      return ad;
    },null,null)),
    (l()(),jit_textDef3(null,['Hello']))
  ]
  ,null,null);
}

1) When the application runs:

function resolveViewDefinition(factory) {
    var value = ((VIEW_DEFINITION_CACHE.get(factory))); // If not cached yet 
    if (!value) { // value is undefined 
        value = factory(function () { return NOOP; }); // Retrieve ViewDefinition without logging errors
        value.factory = factory; // Save reference to the factory for future use
        VIEW_DEFINITION_CACHE.set(factory, value); // Store the factory
    }
    return value;
}

2) Upon clicking on Hello:

((logViewDef.factory))(nodeLogger); // Invoke the factory using the stored reference with logger

https://i.sstatic.net/1gXnN.gif

Answer 2

resolveViewDefinition function is primarily used to retrieve view definitions. It first attempts to fetch the value from the cache, and if it is not found in the cache, then it proceeds to call the ViewDefinitionFactory.

The ViewDefinitionFactory requires a function as a parameter. But why?

/**
 * Factory for ViewDefinitions.
 * We use a function so we can reexecute it in case an error happens and use the given logger
 * function to log the error from the definition of the node, which is shown in all browser
 * logs.
 */
export interface ViewDefinitionFactory { (logger: NodeLogger): ViewDefinition; }

If we are simply obtaining the ViewDefinition, there's no need to log errors, so it is invoked with the NOOP function.

However, in cases where an error occurs during the execution of actions within the template while Angular is running, this factory utilizes the NodeLogger to pinpoint the node responsible for the error.

Check out this example on Plunker

@Component({
  selector: 'my-app',
  template: `<h2 (click)="x()">Hello</h2>`
})
export class App {}

Here is a snippet of the factory code:

function View_App_0(l) {
  return jit_viewDef1(0,[
      (l()(),jit_elementDef2(0,null,null,1,'h2',[],null,[[
        null,
        'click'
      ]
    ],function(v,en,$event) {
      var ad = true;
      var co = v.component;
      if (('click' === en)) {
        var pd_0 = (co.x() !== false);
        ad = (pd_0 && ad);
      }
      return ad;
    },null,null)),
    (l()(),jit_textDef3(null,['Hello']))
  ]
  ,null,null);
}

1) When the application runs:

function resolveViewDefinition(factory) {
    var value = ((VIEW_DEFINITION_CACHE.get(factory))); // If not cached yet 
    if (!value) { // value is undefined 
        value = factory(function () { return NOOP; }); // Retrieve ViewDefinition without logging errors
        value.factory = factory; // Save reference to the factory for future use
        VIEW_DEFINITION_CACHE.set(factory, value); // Store the factory
    }
    return value;
}

2) Upon clicking on Hello:

((logViewDef.factory))(nodeLogger); // Invoke the factory using the stored reference with logger

https://i.sstatic.net/1gXnN.gif

Setting up a variable with no operation assigned to it

Answer №1

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