1. A computer-implemented method comprising: identifying a first set of initial expressions and a second set of initial expressions in computer code requested to be sent from a server system to a client device remote from the server system, wherein the first set of initial expressions and the second set of initial expressions each comprise one or more expressions, are distinct from one another and each comprise potentially sensitive information, and wherein the first set of initial expressions further comprises one or more expressions of instrumentation code for identifying malicious activity; creating a first plurality of replacement expressions arranged in a first order that, when executed in the first order, provides a first result that is equivalent to a result provided by execution of the first set of initial expressions; creating a second plurality of replacement expressions arranged in a second order that, when executed in the second order, provides a second result that is equivalent to a result provided by execution of the second set of initial expressions, wherein the first plurality of replacement expressions are distinct from the second plurality of replacement expressions; generating modified computer code that interferes with an ability of malware to interact with the computer code at the client device by interleaving the first plurality of replacement expressions and the second plurality of replacement expressions with a third set of initial expressions in the computer code to be sent to the client device, wherein the third set of initial expressions excludes the first set of initial expressions and the second set of initial expressions, wherein the interleaving includes: inserting expressions of the first plurality of replacement expressions between expressions of the third plurality of initial expressions such that a first expression of the first plurality of replacement expressions is positioned between a first pair of expressions of the third set of initial expressions, and a second expression of the first plurality of replacement expressions is positioned between a second pair of expressions of the third set initial expressions, wherein the first pair is different from the second pair; and inserting expressions of the second plurality of replacement expressions between expressions of the third plurality of initial expressions, wherein the first plurality of replacement expressions appear in the modified computer code in the first order and the second plurality of replacement expressions appear in the modified computer code in the second order; and providing the modified computer code to the client device; wherein the method is performed by one or more computing devices.