How do I test if a string is empty in Objective-C?

You can check if [string length] == 0. This will check if it's a valid but empty string (@"") as well as if it's nil, since calling length on nil will also return 0.

Marc's answer is correct. But I'll take this opportunity to include a pointer to Wil Shipley's generalized isEmpty, which he shared on his blog:

static inline BOOL IsEmpty(id thing) {
return thing == nil
|| ([thing respondsToSelector:@selector(length)]
&& [(NSData *)thing length] == 0)
|| ([thing respondsToSelector:@selector(count)]
&& [(NSArray *)thing count] == 0);
}

You should better use this category:

@implementation NSString (Empty)

    - (BOOL) isWhitespace{
        return ([[self stringByTrimmingCharactersInSet:[NSCharacterSet whitespaceAndNewlineCharacterSet]]length] == 0);
    }

@end

You can have an empty string in two ways:

1) @"" // Does not contain space

2) @" " // Contain Space

Technically both the strings are empty. We can write both the things just by using ONE Condition

if ([firstNameTF.text stringByReplacingOccurrencesOfString:@" " withString:@""].length==0)
{
    NSLog(@"Empty String");
}
else
{
    NSLog(@"String contains some value");
}

Try the following

NSString *stringToCheck = @"";

if ([stringToCheck isEqualToString:@""])
{
   NSLog(@"String Empty");
}
else
{
   NSLog(@"String Not Empty");
}

Based on multiple answers I have created a ready to use category combining @iDevAmit and @user238824 answers.

Specifically it goes in the following order

1. Check for null/nil
2. Check if if string is empty using it's length count.
3. Check if string is white spaces.

Header

//
//  NSString+Empty.h

#import <Foundation/Foundation.h>

NS_ASSUME_NONNULL_BEGIN

@interface NSString (Empty)
- (BOOL)isEmptyOrWhiteSpacesOrNil;
@end

NS_ASSUME_NONNULL_END

Implementation

//
//  NSString+Empty.m

#import "NSString+Empty.h"

@implementation NSString (Empty)

- (BOOL) isWhitespace{
      return ([[self stringByTrimmingCharactersInSet:[NSCharacterSet whitespaceAndNewlineCharacterSet]]length] == 0);
  }

- (BOOL)isEmptyOrWhiteSpacesOrNil {
     if(self == nil || [self isKindOfClass:[NSNull class]] || self.length==0 || [self isWhitespace] == YES) {
            return YES;
       }
      return NO;
  }

@end

/*
 Credits
 1. https://stackoverflow.com/a/24506942/7551807
 2. https://stackoverflow.com/a/1963273/7551807
 */

Usage: of-course the function will never be triggered if your string is null. Case one is there just for extra security. I advice checking for nullability before attempting to use this method.

if (myString) {
  if [myString isEmptyOrWhiteSpacesOrNil] {
     // String is empty
  }
} else {
// String is null
}