refactor: change fallback return value from null to -1 in str2enum

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---

Author: anandkaranubc

lib/node_modules/@stdlib/math/base/special/heaviside/README.md

@@ -214,6 +214,8 @@ The `continuity` parameter may be one of the following values:
 -   `STDLIB_BASE_HEAVISIDE_CONTINUITY_RIGHT_CONTINUOUS`: if `x == 0`, the function returns `1.0`.
 -   `STDLIB_BASE_HEAVISIDE_CONTINUITY_DISCONTINUOUS`: if `x == 0`, the function returns `NaN`.
 
+If not provided a valid `continuity` parameter, the function returns `NaN` for `x == 0`, behaving like the discontinuous case.
+
 ```c
 double stdlib_base_heaviside( const double x, const STDLIB_BASE_HEAVISIDE_CONTINUITY continuity );
 ```

lib/node_modules/@stdlib/math/base/special/heaviside/lib/str2enum.js

@@ -43,7 +43,7 @@ var ENUM = enumeration();
 */
 function str2enum( ctype ) {
 	var v = ENUM[ ctype ];
-	return ( typeof v === 'number' ) ? v : null; // note: we include this guard to prevent walking the prototype chain
+	return ( typeof v === 'number' ) ? v : -1; // note: we include this guard to prevent walking the prototype chain
 }
 
 

lib/node_modules/@stdlib/math/base/special/heaviside/test/test.native.js

@@ -121,6 +121,18 @@ tape( 'if the `continuity` option is `right-continuous`, the function returns `1
 	t.end();
 });
 
+tape( 'if the `continuity` option is not valid, the function returns `NaN` if provided `+-0`', opts, function test( t ) {
+	var v;
+
+	v = heaviside( -0.0, 'foo' );
+	t.strictEqual( isnan( v ), true, 'returns expected value' );
+
+	v = heaviside( +0.0, 'bar' );
+	t.strictEqual( isnan( v ), true, 'returns expected value' );
+
+	t.end();
+});
+
 tape( 'the function returns `NaN` if provided `NaN`', opts, function test( t ) {
 	var v = heaviside( NaN, 'left-continuous' );
 	t.strictEqual( isnan( v ), true, 'returns expected value' );