JavaScript: If you want to check whether a variable is defined, you can use typeof:

Example

if(typeof someVar == 'undefined') {
  document.write("Variable someVar is not defined");
} else {
  document.write("Variable someVar is defined");
}

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JavaScript: If you only want to check whether a defined variable is true, you can directly use the following method:

Example

if (strValue) {
    // Code executed when strValue is true
} else {
    // Code executed when strValue is false
}

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The following uses a regular expression method to check whether a variable is defined and not empty; it is a relatively complete method:

Example

if ( // Return the determined value
        (typeof x == 'undefined')
              ||
        (x == null)
              ||
        (x == false)        // Similar to: !x
              ||
        (x.length == 0)
              ||
        (x == 0)            // Here it checks 0; no need to deliberately remove it
              ||
        (x == "")
              ||
        (x.replace(/\s/g,"") == "")
              ||
        (!/[^\s]/.test(x))
              ||
        (/^\s*$/.test(x))
    ) {
  document.write("Variable is not defined or is empty");
}

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You can also encapsulate a method to check, covering null, 0, false, etc., suitable for defined variables:

Example

function empty(e) {
  switch (e) {
    case "":
    case 0:
    case "0":
    case null:
    case false:
    case undefined:
      return true;
    default:
      return false;
  }
}

empty(null) // true
empty(0) // true
empty(7) // false
empty("") // true
empty((function() {
    return ""
})) // false

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