How to detect boolean true and false in javascript? [duplicate]

Question

This is my code for date validate, when fill 28/02/2017 into input type text and press button.

First it's will alert true when see in code if first alert is true secone alert will alert true too. But my second is alert false.

What is my miskate for detect boolean

if(regex.test(txt) === true)
{
    alert("true");
    p.innerHTML = "<h1 style='color:green'>Correct</h1>";
}
else
{
    alert("false");
    p.innerHTML = "<h1 style='color:red'>Wrong</h1>";;
}

.

<input type="text" id="date" name="date" value="dd/mm/yyyy" />
<button onclick="isDate();">Check</button>

<p id="response"></p>
<script>
function isDate() {
    
    var regex = /^(((0[1-9]|[12]\d|3[01])\/(0[13578]|1[02])\/((19|[2-9]\d)\d{2}))|((0[1-9]|[12]\d|30)\/(0[13456789]|1[012])\/((19|[2-9]\d)\d{2}))|((0[1-9]|1\d|2[0-8])\/02\/((19|[2-9]\d)\d{2}))|(29\/02\/((1[6-9]|[2-9]\d)(0[48]|[2468][048]|[13579][26])|((16|[2468][048]|[3579][26])00))))$/g;
    
    var txt = document.getElementById("date").value;
    var p = document.getElementById("response");
    
	alert(regex.test(txt));
	
	if(regex.test(txt) === true)
	{
		alert("true");
		p.innerHTML = "<h1 style='color:green'>Correct</h1>";
	}
	else
	{
		alert("false");
		p.innerHTML = "<h1 style='color:red'>Wrong</h1>";;
	}  
} 
</script>

For some reason, adding console.log below the alert repeating the test, then removing the === and true it returns true then logs false... VERY strange!! spoookky — L_Church
– L_Church, Commented Apr 26, 2018 at 15:29

Rory O'Kane · Accepted Answer · 2018-04-26 15:48:02Z

7

The reason is that regex.test() will search the string and return true if there is a match. If you then subsequently call .test() again it will continue from where it left off.

Using test() on a regex with the global flag

If the regex has the global flag set, test() will advance the lastIndex of the regex. A subsequent use of test() will start the search at the substring of str specified by lastIndex (exec() will also advance the lastIndex property).

From here: MDN web docs – RegExp.prototype.test()

edited Apr 26, 2018 at 15:48

Rory O'Kane

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answered Apr 26, 2018 at 15:32

Reinstate Monica Cellio

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7 Comments

reswe teryuio Over a year ago

work good sir..

Albeis Over a year ago

Just a question, why it is working if you remove g modifier? thanks!

Reinstate Monica Cellio Over a year ago

@Albeis If you remove the global flag it searches from the beginning every time.

Albeis Over a year ago

Thanks! so removing global flag should be another solution right? regards

Rory O'Kane Over a year ago

@Albeis ‘g’ stands for ‘global’ – the g modifier sets the global flag. And that affects the “If the regex has the global flag set” behavior described in the documentation.

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