The expression `Object.prototype.toString.call(currentFruit) === "[object Date]"` checks if the object referenced by `current

Question

The expression `Object.prototype.toString.call(currentFruit) === "[object Date]"` checks if the object referenced by `current

Hi, I'm just starting to learn JavaScript and I have a question about an if condition that I came across in my code. Can someone please explain what this specific if condition does?

Object.prototype.toString.call(currentFruit) === "[object Date]"

I have also included the relevant parts of my code for reference:

setcurrentFruit: function (fruitName, currentFruit) {
    WorklistStorage.set(fruitName, currentFruit, false);
},
getcurrentFruit: function (fruitName) {
    var currentFruit = unescapeJSON(WorklistStorage.get(fruitName, false));
    if (currentFruit == "undefined" || typeof currentFruit == "undefined" || Object.prototype.toString.call(currentFruit) === "[object Date]") {
        var date = new Date();
        currentFruit = date.toString();
        wholeQueue.setcurrentFruit(fruitName, currentFruit);
        //console.log("poppppp");
    }
    currentFruit = new Date(currentFruit);
    return currentFruit;
},

javascript html jquery css jquery-events

Answer 1

Answer №1

Breaking it down:

Object; the go-to resource for native JavaScript Objects.
Object.prototype; the reference to properties inherited by all instances of Object.
Object.prototype.toString; the built-in toString method of all Objects.
Function.prototype.call; a way to execute a function with a specified this

Hence,

Object.prototype.toString.call(currentFruit)

invokes the standard toString function of all Objects on currentFruit. This can vary from currentFruit.toString() if there is another custom toString defined on or inherited by currentFruit.

Object.prototype.toString gives back a string in the format [object X] where X represents the type of this, therefore checking against [object Date] with === is essentially asking: "Is currentFruit a Date?"

Why opt for this check over typeof? Because typeof often just returns "object", which isn't always informative.

And what about instanceof? It evaluates to true if the object being checked inherits from what you're testing against as well, hence x instanceof Object usually returns true, which isn't consistently helpful either.

Another approach similar to consider would be examining the constructor of an Object. x.constructor === Date. While this poses different challenges like throwing errors if x is undefined or null requiring additional checks, it could be more beneficial when dealing with non-native constructors, where toString might just provide [object Object].

With that said, ponder whether this test will ever hold true within your working context. As of now, there isn't a universally accepted JSON representation of a Date.

Answer 2

Breaking it down:

Object; the go-to resource for native JavaScript Objects.
Object.prototype; the reference to properties inherited by all instances of Object.
Object.prototype.toString; the built-in toString method of all Objects.
Function.prototype.call; a way to execute a function with a specified this

Hence,

Object.prototype.toString.call(currentFruit)

invokes the standard toString function of all Objects on currentFruit. This can vary from currentFruit.toString() if there is another custom toString defined on or inherited by currentFruit.

Object.prototype.toString gives back a string in the format [object X] where X represents the type of this, therefore checking against [object Date] with === is essentially asking: "Is currentFruit a Date?"

Why opt for this check over typeof? Because typeof often just returns "object", which isn't always informative.

And what about instanceof? It evaluates to true if the object being checked inherits from what you're testing against as well, hence x instanceof Object usually returns true, which isn't consistently helpful either.

Another approach similar to consider would be examining the constructor of an Object. x.constructor === Date. While this poses different challenges like throwing errors if x is undefined or null requiring additional checks, it could be more beneficial when dealing with non-native constructors, where toString might just provide [object Object].

With that said, ponder whether this test will ever hold true within your working context. As of now, there isn't a universally accepted JSON representation of a Date.

Answer 3

Answer №2

Object.prototype.toString function is utilized to retrieve the internal [[Class]] value of javascript objects. This is particularly useful in verifying whether currentFruit is a standard Date object.

An alternative, albeit with slight nuances, would be to use currentFruit instanceof Date.

Answer 4

Object.prototype.toString function is utilized to retrieve the internal [[Class]] value of javascript objects. This is particularly useful in verifying whether currentFruit is a standard Date object.

An alternative, albeit with slight nuances, would be to use currentFruit instanceof Date.

The expression `Object.prototype.toString.call(currentFruit) === "[object Date]"` checks if the object referenced by `current

Answer №1

Answer №2

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