基本的如果语句脑放屁HTML javascript

Question

trying to learn javascript for the first time i am stuck on an issue involving an if statement I want the user to be able to put in the text a letter then have the computer say whether what he put in was right or wrong when I run this it usually skips some part of the if statement. this could be because I'm using the != symbol but I don't know below is the code to show you what I am working with I hope I can get you some help!

 document.getElementById("Submit").onclick = function(){ var a = "'a', 'b', 'c'"; var enter = document.getElementById("text").value; a.toString(); if (a == enter) { alert("congrats"); } else if(enter == null) { alert("you put nothing"); } else if (enter != a) { alert("wrong try again") } else { alert("End of if statment") } } 

 <form> First name:<br> <input type="text" id = "text" name="firstname"> </form> <button type="button" id ="Submit">Submit!</button> 

Answer 1

First, you have to change your a variable. Now you have an array(3 different values a,b,c), or just weird string( 'a','b','c'), I'm not sure about this. It shuold look like this:

var a = "abc";

Now, your "congrats" alert should work, as it can actually try to weak-compare these two values succesfully.

Next step you have to make, is to change null for "".

else if (enter == "")

That's because this value(no input) is actually undefined value, not null, so with "null" there is no way to compare this.

These are just basic primitive variables types. For more information I would reccomend to read this: Read That Book

Answer 2

You declared a as a string with value 'a', 'b', 'c' so it is a string with some letters and the ' symbol.

If you want to check if what the user entered is one of those letters declared by you (let's say a , b or c )...

You should (for your attempt) declare it as an array like this: var a = ['a', 'b', 'c'] and use Array.prototype.indexOf in order to see if what the user entered is a letter inside the array.

See below snippet

 document.getElementById("SubmitArrayVersion").onclick = function() { var aArray = ['a', 'b', 'c']; // array version var enter = document.getElementById("text").value; if (aArray.indexOf(enter) !== -1) { alert("congrats"); } else if (!enter) { alert("you put nothing"); } else { alert("wrong try again") } } 

 First name:<br> <input type="text" id="text" name="firstname"> </form> <button type="button" id="SubmitArrayVersion">Submit! (Array version)</button> 

Answer 3

It will be correct when the user enters 'a', 'b', 'c' into the input field.

If you try the code below

 document.getElementById("Submit").onclick = function(){ var array = ['a', 'b', 'c']; var enter = document.getElementById("text").value; if (array[0] == enter){ alert("congrats"); } else if(enter == null){ alert("you put nothing"); } else if (enter != array[0]){ alert("wrong try again") } else{ alert("End of if statment") } } 

 <form> First name:<br> <input type="text" id = "text" name="firstname"> </form> <button type="button" id ="Submit">Submit!</button> 

That will alert congrats when a is entered into the input as it is checking the first element of the array (assuming if you have worked with arrays)

Answer 4

Assuming you want your program to print out success when the user puts in either the single letter a, b, or c this should work.

 document.getElementById("Submit").onclick = function() { var enter = document.getElementById("text").value; if ('a' === enter || 'b' === enter || 'c' === enter) { alert("congrats"); } else if (enter == "") { alert("you put nothing"); } else if('a' != enter || 'b' != enter || 'c' != enter) { alert("wrong try again"); } } 

 <form> First name:<br> <input type="text" id="text" name="firstname"> </form> <button type="button" id="Submit">Submit!</button> 

In the first if statement I used the || operator to distinguish between different conditions, the expression in the if statement will convert to true if enter is either the string a, b, or c.

Then the second part checks if enter is empty, the third part is kinds obsolete because we already know that enter is not a, b, or c because we checked it in the first if statement but I left it in for reference.

So:

  } else if('a' != enter || 'b' != enter || 'c' != enter) {
    alert("wrong try again");
  }

Could be rewritten to:

else {
   alert("wrong try again");
}