Underscore.js Differences

_.m aims to be as compatible with Underscore.js as possible, but some differences exist due to language differences between JavaScript and Objective-C.

Not Supported

The following Underscore.js functions are not available in _.m:

Function	Reason
bind	Objective-C blocks capture scope automatically
bindAll	Not applicable
isElement	No DOM in Objective-C
isRegExp	Different regex API
isUndefined	Objective-C has nil instead
noConflict	Not applicable
mixin	Different extension model
escape	Different string escaping needs
template	Different templating approach

No Context Parameter

In Underscore.js, many functions accept an optional context parameter:

// JavaScript
_.each(list, iterator, context);

In _.m, blocks automatically capture their surrounding scope, so context is not needed:

// Objective-C - context captured automatically
__weak typeof(self) weakSelf = self;
_.each(list, ^(id item, ...) {
    [weakSelf doSomething:item];
});

Type Differences

JavaScript	Objective-C
undefined	nil
null	nil or [NSNull null]
Array	NSArray
Object	NSDictionary
Function	Block (^)

Variadic Arguments

_.m uses ... (va_list) for optional iterator arguments:

// Block signature includes ... for optional key and list
_.each(list, ^(id value, ...) {
    // Access optional args via va_list if needed
});

Helper Macros

_.m provides macros for extracting variable arguments in blocks:

Collection Iteration (no terminators needed)

Macro	Description
ARGS_KEY(value)	Extracts the key (second argument)
ARGS_INDEX(value)	Extracts the index (second argument)
ARGS_LIST(value, LIST_NAME)	Extracts the list (third argument)

Variable Arguments (terminators required)

Macro	Description
ARGS_AO(NAME, lastNamedArg)	Collects all id objects after the specified argument (nil-terminated)
ARGS_AI(NAME, lastNamedArg)	Collects all integers after the specified argument (AI_END-terminated)

Single Argument Extraction

Macro	Description
ARG_B()	Extract one BOOL
ARG_I()	Extract one NSInteger
ARG_UI()	Extract one NSUInteger
ARG_F()	Extract one float
ARG_D()	Extract one double
ARG_N()	Extract one NSNumber*
ARG_NSO()	Extract one NSObject*

Block Type Definitions

_.m defines block types for consistency:

Block Type	Signature	Usage
_EachBlock	^(id value, ...)	Iteration without return
_EachWithStopBlock	^B(id value, ...)	Returns boolean to control loop
_MapBlock	^id(id value, ...)	Returns transformed value
_ReduceBlock	^id(id memo, id value, ...)	Accumulation function
_FindBlock	^B(id value)	Predicate for finding
_ItemTestBlock	^B(id value, ...)	Predicate for filtering
_MaxBlock / _MinBlock	^id(id value)	Comparison function
_SortByBlock	^id(id value, ...)	Returns sort key
_GroupByBlock	^id(id value, ...)	Returns grouping key
_DelayBlock / _DeferBlock	^()	No arguments, no return
_ThrottleBlock	^id()	Returns id
_DebounceBlock	^(id arg1, ...)	Variable arguments
_OnceBlock	^id(id arg1, ...)	Execute once
_AfterBlock	^id(id arg1, ...)	Execute after N calls
_WrapBlock	^id(_WrappedBlock wrapped, id arg1, ...)	Wraps another block
_ComposeBlock	^id(id arg1, ...)	Function composition
_TapBlock	^(id obj)	Side-effect interceptor
_TimesBlock	^(I index)	Iteration N times
_IdentityBlock	^id(id value)	Returns unchanged value

Memory Management

_.m is fully ARC compatible. When capturing self in blocks, use __weak to avoid retain cycles:

__weak typeof(self) weakSelf = self;
_.each(items, ^(id item, ...) {
    [weakSelf processItem:item];
});

Type Shortcuts

_.m provides shorthand type aliases for cleaner code:

// Instead of
NSArray* result = ...;
NSDictionary* dict = ...;

// Use
A* result = ...;
O* dict = ...;

See the Introduction for the full list of type shortcuts.