CODEWITHSUNDEEP
javascript
Thallius
Thallius

Reputation: 2619

Why do this dates not match?

i have this code which works with dates:

let startDate = new Date(toolOrder.jsAppStartDate.getTime()); // clone date
startDate.setDate(startDate.getDate() - Math.floor((days - 1) / 2)); // sub some days
console.log(toolOrder.jsAppStartDate); // Output: Date Thu Sep 21 2023 00:00:00 GMT+0200 (Central European Summer Time)
console.log(toolOrder.jsAppStartDate.getTime()); // Output: 1695247200000
console.log("--------------");
for (let index = 0; index < days; index++)
{
    console.log(startDate);
    console.log(startDate.getTime());
    // ... some drawing code here ...
    startDate.setDate(startDate.getDate() + 1); // add one day
    if(startDate === toolOrder.jsAppStartDate) // check if start date reached
    {
        console.log("Dates match");
        startDate = new Date(toolOrder.jsAppEndDate.getTime());
    }
}

The output of the log before the loop is:

Date Thu Sep 21 2023 00:00:00 GMT+0200 (Central European Summer Time)
1695247200000
---------------

The output from the loop is:

Date Mon Sep 18 2023 00:00:00 GMT+0200 (Central European Summer Time)
1694988000000 
Date Tue Sep 19 2023 00:00:00 GMT+0200 (Central European Summer Time)
1695074400000
Date Wed Sep 20 2023 00:00:00 GMT+0200 (Central European Summer Time)
1695160800000
Date Thu Sep 21 2023 00:00:00 GMT+0200 (Central European Summer Time)
1695247200000
Date Fri Sep 22 2023 00:00:00 GMT+0200 (Central European Summer Time)
1695333600000

So even if the dates match in loop 3, the if condition is not true. It works when I use

if(startDate.getTime() === toolOrder.jsAppStartDate.getTime())

but I thought it should work without also?

Upvotes: 0

Views: 104

Answers (2)

Marco
Marco

Reputation: 7261

=== compares identity so:

startDate === toolOrder.jsAppStartDate

Means: is startDate the same object [in memory] as toolOrder.jsAppStartDate which is not the case because toolOrder.jsAppStartDate is a different object and not the object referred to by the name startDate.

mdn says this about the strict equality operator:

If both operands are objects, return true only if they refer to the same object.

(emphasis mine)

if(startDate.getTime() === toolOrder.jsAppStartDate.getTime())

Works because you're now comparing numbers instead of objects.

The rule for the two operands being numbers is:

Otherwise, compare the two operand's values:

Numbers must have the same numeric values. +0 and -0 are considered to be the same value.

mdn (emphasis mine)

Upvotes: 2

Mehmet Baker
Mehmet Baker

Reputation: 1155

You are comparing Date instances with each other when you're using the === operator. However, they are not the same instance. Each new Date creates a unique, new instance (unless the class constructor deliberately returns a previously created instance [for more info: singleton pattern]).

When you are comparing each instance's getTime() result, you are comparing 2 primitive values (numbers in this case). Primitive values are not class instances; so === works like == with primitives.

Upvotes: 2

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