Ejemplo 1: javascript comprobar si no está definido o es nulo
if( typeof myVar === 'undefined' || myVar === null ){
// myVar is undefined or null
}
Ejemplo 2: javascript comprobar si no es nulo
//Even if the value is 0, this will execute.
if (myVar !== null) {...}
//If you don't want it to execute when it's 0, then set it as
if (myVar) {...}
//This will return false if var value is 0.
Ejemplo 3: javascript comprobar si es indefinido o nulo o cadena vacía
// simple check do the job
if (myString) {
// comes here either myString is not null,
// or myString is not undefined,
// or myString is not '' (empty).
}
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